bump-formula-pr: cleanup

[SeekingMeaning]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

---

This PR includes some general cleanup of brew bump-formula-pr and cleanup of #10221

• Deleted requested_spec variable since its value is always :stable
• Moved the check for stable specification to happen earlier
• Added some missing .present?
• Fixed strange indentation of check_closed_pull_requests call
• Replaced duplicate file reads with old_contents

[BrewTestBot]

Review period will end on 2021-01-12 at 00:00:00 UTC.

[BrewTestBot]

Review period ended.