DEV: Remove .github/workflows/check-branches.yml (#24331)

We no longer need this workflow.

.github/workflows/check-branches.yml

@@ -1,15 +0,0 @@
-name: PR base branch check
-on:
-  pull_request:
-jobs:
-  check-branches:
-    if: github.event_name == 'pull_request' && github.repository == 'discourse/discourse-private-mirror'
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check branches
-        run: |
-          BASE_REF=${{ github.base_ref }}
-          if [ "$BASE_REF" == "tests-passed" ]; then
-            echo "PR requests to tests-passed branch are not allowed. Please use main."
-            exit 1
-          fi