github: don't checkout the head of a pull request

addresses this annotation:
1 issue was detected with this workflow: git checkout HEAD^2 is no
longer necessary. Please remove this step as Code Scanning recommends
analyzing the merge commit for best results.

.github/workflows/codeql-analysis.yml

@@ -19,15 +19,6 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3
-      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