Minor fix in flow 'Merge when build succeeds'

When a user, which is not the merge user, want to removes the source branch
after the automatic merge this might fail because of checks in the
DeleteBranchService

app/views/projects/merge_requests/widget/open/_merge_when_build_succeeds.html.haml

@@ -12,7 +12,7 @@
     - else
       The source branch will not be removed.
 
-  - remove_source_branch_button = @merge_request.can_remove_source_branch?(current_user) && !should_remove_source_branch
+  - remove_source_branch_button = @merge_request.can_remove_source_branch?(current_user) && !should_remove_source_branch && @merge_request.merge_user == current_user
   - user_can_cancel_automatic_merge = @merge_request.can_cancel_merge_when_build_succeeds?(current_user)
   - if remove_source_branch_button || user_can_cancel_automatic_merge
     .clearfix.prepend-top-10