Revert #1311 - false negatives for `Rails/ActionControllerFlashBeforeRender`

[Earlopain]

Relates to #1269.

This cop would need to do control flow analysis which it just doesn't do. RuboCop also has no mechanism for that.

So just reverting this for now to fix the newly introduces false positives. No "Fix" in the commit since the original problem from the issue still occurs.

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Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• [ ] If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

This looks good to me. Can you squash your commits into one?