Exclude controllers and mailers from Lint/UselessMethodDefinition

[r7kamura]

This is a proposal to address the following issues:

• Lint/UselessMethodDefinition has a conflict with Rails/LexicallyScopedActionFilter when used with Rails before_action rubocop#8667
• Cop Rails/LexicallyScopedActionFilter conflicts with Lint/UselessMethodDefinition #343

Currently, Rails/LexicallyScopedActionFilter is enabled by default for controllers and mailers.

rubocop-rails/config/default.yml

Lines 670 to 678 in f57c3ba

	Rails/LexicallyScopedActionFilter:
	Description: "Checks that methods specified in the filter's `only` or `except` options are explicitly defined in the class."
	StyleGuide: 'https://rails.rubystyle.guide#lexically-scoped-action-filter'
	Enabled: true
	Safe: false
	VersionAdded: '0.52'
	Include:
	- '**/app/controllers/**/*.rb'
	- '**/app/mailers/**/*.rb'

Given this, I thought it would be reasonable to disable Lint/UselessMethodDefinition by default for them as well.

---

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• 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.