Update CodeQL workflow

Bring the CodeQL workflow up to date with the latest recommended
configuration, which analyses the merge commit of pull requests (not the
head of the PR branch).

.github/workflows/codeql-analysis.yml

@@ -14,16 +14,7 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2
-      with:
-        # We must fetch at least the immediate parents so that if this is
-        # a pull request then we can checkout the head.
-        fetch-depth: 2
 
-    # If this run was triggered by a pull request event, then checkout
-    # the head of the pull request instead of the merge commit.
-    - run: git checkout HEAD^2
-      if: ${{ github.event_name == 'pull_request' }}
-      
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL
       uses: github/codeql-action/init@v1