Remove the obsolete `requires_approve?` method

The following discussion from !5635 (merged) should be addressed:

• @rdavila started a discussion: (+3 comments)

  If approvals_required is 0 is a bit confusing having requires_approve? to return true, can we improve that?

!5635 (merged) removes the ability to disable approvals for a merge request, in favour of being able to set the number of approvals to 0. This ~technical debt issue tracks the process of removing some legacy code that the frontend depends on to conditionally render certain elements. At present, the backend is simply returning true unconditionally.

/cc @crollison