github: Remove obsolete LGTM workflow

lgtm.com will be shutdown soon.

.github/workflows/lgtm.yml

@@ -1,37 +0,0 @@
-name: lgtm.com
-
-on: [push]
-
-env:
-  OS_NAME: linux
-
-jobs:
-  pre-check:
-    runs-on: ubuntu-latest
-    outputs:
-      should_skip: ${{ steps.skip-check.outputs.should_skip }}
-    steps:
-      - id: skip-check
-        uses: fkirc/skip-duplicate-actions@master
-        with:
-          concurrent_skipping: 'same_content'
-
-  lgtm:
-    needs: pre-check
-    if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
-    runs-on: ubuntu-latest
-    env:
-      TEST: lgtm
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          fetch-depth: 0
-      # we don't use github/codeql-action because we can't exclude queries there,
-      # so we continue to use the approach we used on Travis
-      - env:
-          LGTM_TOKEN: ${{ secrets.LGTM_TOKEN }}
-          LGTM_PROJECT: ${{ secrets.LGTM_PROJECT }}
-          BUILD_NUMBER: ${{ github.run_number }}
-          COMMIT_ID: ${{ github.sha }}
-          COMMIT_BASE: ${{ github.event.before }}
-        uses: ./.github/actions/default