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.

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