Remove duplicate calls of MergeRequest#can_be_reverted? on MR widget endpoint

What does this MR do?

According to http://profiler.gitlap.com/20180123/d988e1eb-0370-4a25-9a4e-6d36112a403a.html.gz, we can see Commit#has_been_reverted? is called twice in MergeRequestsController#show.json. That happens because MergeRequest#can_be_reverted? is also called twice on MergeRequestsPresenter.

Reported via https://gitlab.com/gitlab-org/gitlab-ce/issues/36876#note_55858440

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?