Remove Style/ExtendSelf cop

[pocke]

Style/ExtendSelf was introduced by #5108, but the cop duplicates with Style/ModuleFunction.
So this change reverts the commit (da69b88).

---

Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

That's new. 🤣 Did you compare which one was implemented better?

[pocke]

I didn't compare them. I thought the new cop should be removed for compatibility, so I removed it. Maybe we should work on another pull-request to improve the cop's implementation.

[Drenmi]

Whoops! 😬

[bbatsov]

I didn't compare them. I thought the new cop should be removed for compatibility, so I removed it. Maybe we should work on another pull-request to improve the cop's implementation.

@Drenmi Can you look into this? I wonder how you didn't notice the existing cop. 😆 I'd assumed you'd be getting some offenses from it as well. Perhaps you're just doing TDD. 😉

[flyerhzm]

Style/ExtendSelf has an autocorrect method while Style/ModuleFunction does not, looks we should remove Style/ModuleFunction cop instead.

[pocke]

Style/ExtendSelf has an autocorrect method while Style/ModuleFunction does not, looks we should remove Style/ModuleFunction cop instead.

Maybe we can move the auto-correction to Style/ModuleFunction. 👍
And I think we can't remove Style/ModuleFunction, because some users configure the gem already. So if remove the cop, it's a breaking change.