Drop check_branch CI job

Now that GitHub Actions always skips any job if [ci skip] is given
regardless of our CI config, we cannot merge such PRs like https://github.com/ruby/ruby/pull/4211
unless you're an admin.
https://github.blog/changelog/2021-02-08-github-actions-skip-pull-request-and-push-workflows-with-skip-ci/

To avoid letting us spend time to ask contributors to remove [ci skip]
before merging PRs, I removed the "check_branch" requirement from
protected branches.

Because the main purpose of this job was to prevent (non-admin)
committers from directly pushing commits to the GitHub mirror, this job
is now useless.

.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 }}