Remove check branch from ruby_3_0

.github/workflows/check_branch.yml

@@ -1,22 +0,0 @@
-# We bidirectionally synchronize github.com/ruby/ruby.git's master branch and
-# git.ruby-lang.org/ruby.git's master branch.
-# We can use a pull request's merge button only on the master branch.
-#
-# Therefore, we require to pass this "check_branch" on all protected branches
-# to prevent us from accidentally pushing commits to GitHub directly.
-#
-# Details: https://bugs.ruby-lang.org/issues/16094
-name: Pull Request
-on: [pull_request]
-jobs:
-  check_branch:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Check if branch is master
-        run: |
-          if [ "$BASE_REF" != master ]; then
-            echo "Only master branch accepts a pull request, but it's '$BASE_REF'."
-            exit 1
-          fi
-        env:
-          BASE_REF: ${{ github.base_ref }}