codeql-analysis.yml: remove git checkout HEAD^2.

According to CodeQL instruction, `HEAD^2` is no longer needed.

.github/workflows/codeql-analysis.yml

@@ -19,11 +19,6 @@ jobs:
           # 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