Check remove source branch when merge is in progress

app/views/projects/merge_requests/merge.js.haml

 - case @status
 - when :success
+  - remove_source_branch = params[:should_remove_source_branch] == '1' || @merge_request.remove_source_branch?
   :plain
-    merge_request_widget.mergeInProgress(#{params[:should_remove_source_branch] == '1'});
+    merge_request_widget.mergeInProgress(#{remove_source_branch});
 - when :merge_when_pipeline_succeeds
   :plain
     $('.mr-widget-body').html("#{escape_javascript(render('projects/merge_requests/widget/open/merge_when_pipeline_succeeds'))}");