Swift: Drop support for plaintext diagnostics (and helpLinks).

The recommended option is Markdown diagnostics, and we have already migrated everything to emit them. The empty help link we're currently emitting everywhere is a bug.

swift/integration-tests/linux-only/autobuilder/unsupported-os/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "Currently, `autobuild` for Swift analysis is only supported on macOS.\n\n[Change the Actions runner][1] to run on macOS.\n\nYou may be able to run analysis on Linux by setting up a [manual build command][2].\n\n[1]: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#jobsjob_idruns-on\n[2]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/failure/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "`autobuild` failed to run the detected build command:\n\n```\n/usr/bin/xcodebuild build -project <test-root-directory>/hello-failure.xcodeproj -target hello-failure CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO\n```\n\nSet up a [manual build command][1] or [check the logs of the autobuild step][2].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language\n[2]: https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/using-workflow-run-logs",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/no-build-system/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "`autobuild` could not detect an Xcode project or workspace.\n\nSet up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/no-swift-with-spm/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "A Swift package was detected, but no viable Xcode target was found.\n\nSwift Package Manager builds are not currently supported by `autobuild`. Set up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/no-swift/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "All targets found within Xcode projects or workspaces either contain no Swift source files, or are tests.\n\nTo analyze a custom set of source files, set up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/no-xcode-with-spm/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "A Swift package was detected, but no viable Xcode target was found.\n\nSwift Package Manager builds are not currently supported by `autobuild`. Set up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/only-tests-with-spm/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "A Swift package was detected, but no viable Xcode target was found.\n\nSwift Package Manager builds are not currently supported by `autobuild`. Set up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {

swift/integration-tests/osx-only/autobuilder/only-tests/diagnostics.expected

@@ -1,7 +1,4 @@
 {
-  "helpLinks": [
-    ""
-  ],
   "markdownMessage": "All targets found within Xcode projects or workspaces either contain no Swift source files, or are tests.\n\nTo analyze a custom set of source files, set up a [manual build command][1].\n\n[1]: https://docs.github.com/en/enterprise-server/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages#adding-build-steps-for-a-compiled-language",
   "severity": "error",
   "source": {