Partially revert "Revert "Merge branch 'gitaly-mandatory-20180703-jv' into 'master'""

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20402 we reverted more than we had to. This re-applies the changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20339 that did not break gitlab-ee.

Removes Gitaly feature flags: gitaly_find_commit, gitaly_conflicts_list_conflict_files, gitaly_conflicts_resolve_conflicts.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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