Revert "Swift: auto-flush logs at exit"

This reverts commit 0d9dcb161f.

This turns out to introduce a subtle bug related to destruction order
between `Log::instance()` and the `Logger` instances.

swift/xcode-autobuilder/XcodeBuildRunner.cpp

@@ -72,6 +72,7 @@ void buildTarget(Target& target, bool dryRun) {
     if (!exec(argv)) {
       DIAGNOSE_ERROR(build_command_failed, "The detected build command failed (tried {})",
                      absl::StrJoin(argv, " "));
+      codeql::Log::flush();
       exit(1);
     }
   }