Merge branch 'cleanup-7180' into 'master'

A simpler implementation of finding a merge request

## What does this MR do?

This cleans up some code in the issues API as discussed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7180#note_19501743

The helper method `merge_request_for_resolving_discussions` was no longer needed as it was only used once. That made it possible to simplify the implementation.

## Does this MR meet the acceptance criteria?

- [x] ~~[Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added~~
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

!7180

See merge request !7955

lib/api/issues.rb

@@ -28,14 +28,6 @@ module API
 
         new_params
       end
-
-      def merge_request_for_resolving_discussions
-        return unless merge_request_iid = params[:merge_request_for_resolving_discussions]
-
-        @merge_request_for_resolving_discussions ||= MergeRequestsFinder.new(current_user, project_id: user_project.id).
-                                                       execute.
-                                                       find_by(iid: merge_request_iid)
-      end
     end
 
     resource :issues do
@@ -179,7 +171,12 @@ module API
         attrs = attributes_for_keys(keys)
 
         attrs[:labels] = params[:labels] if params[:labels]
-        attrs[:merge_request_for_resolving_discussions] = merge_request_for_resolving_discussions if params[:merge_request_for_resolving_discussions]
+
+        if merge_request_iid = params[:merge_request_for_resolving_discussions]
+          attrs[:merge_request_for_resolving_discussions] = MergeRequestsFinder.new(current_user, project_id: user_project.id).
+            execute.
+            find_by(iid: merge_request_iid)
+        end
 
         # Convert and filter out invalid confidential flags
         attrs['confidential'] = to_boolean(attrs['confidential'])