Remove duplicate merge request approval APIs

The following discussion from !9633 (merged) should be addressed:

• @yorickpeterse started a discussion: (+1 comment)

  @mdelaossa You seem to have worked on these approval related APIs recently. Looking at the code, API::MergeRequests and API::MergeRequestApprovals appear to be quite similar, both having endpoints for approving merge requests and such.

  Do you happen to know why this is, and why API::MergeRequestApprovals appears to be order dependant?

Backstory: in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9633 we found out that there are two APIs for merge request approvals:

1. API::MergeRequests
2. API::MergeRequestApprovals

Both seem to offer the same endpoints, but use slightly different code to do so. We should determine which API to keep, and which one can then be removed.