[Fix #1249] Disable Rails/UnusedIgnoredColumns by default

[Earlopain]

Fix #1249

This cop can't be used to effectivly check for unused columns because the development and production schema can be out of sync until the migration has been run on production.
As such, this cop can cause ignored_columns to be removed even though the production schema still contains the column, which can lead to downtime when the migration is actually executed.

Users can manually enable this cop if they know this isn't an issue that can/will impact them.

cc @rafaelfranca, let me know if the added tidbit to the cops documentation is not correct.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• 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.

[rafaelfranca]

Thank you for working on this. The note added to the cop makes sense to me

[koic]

@pocke Do you have any comments on this proposal? If there are no objections, I will proceed with the merge.