API: Make the /notes endpoint work with noteable iid instead of id

What does this MR do?

It cleans up some things that were forgotten during the implementation of API V4:

• Remove the newly added GET /projects/:id/merge_requests/:merge_request_iid/comments and POST /projects/:id/merge_requests/:merge_request_iid/comments endpoints
• Change GET /projects/:id/merge_requests/:merge_request_id/notes and POST /projects/:id/merge_requests/:merge_request_id/notes to GET /projects/:id/merge_requests/:merge_request_iid/notes and POST /projects/:id/merge_requests/:merge_request_iid/notes (i.e. merge_request_iid instead merge_request_id), same
• Keep GET /v3/projects/:id/merge_requests/:merge_request_id/notes and POST /v3/projects/:id/merge_requests/:merge_request_id/notes

Ported to EE in gitlab-org/gitlab-ee!1518

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#29871