Fix warning "git checkout HEAD^2 is no longer necessary" (#666)

Author: nakamura-to

.github/workflows/codeql-analysis.yml

@@ -12,11 +12,6 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
-        with:
-          fetch-depth: 2
-
-      - run: git checkout HEAD^2
-        if: ${{ github.event_name == 'pull_request' }}
 
       - name: Initialize CodeQL
         uses: github/codeql-action/init@v1