Allow bulk-updating to remove all labels

Instead of passing `remove_label_ids`, just pass an empty array for
`label_ids` (and don't pass `add_label_ids` or `remove_label_ids`).

app/services/issues/bulk_update_service.rb

@@ -4,7 +4,7 @@ module Issues
       issues_ids   = params.delete(:issues_ids).split(",")
       issue_params = params
 
-      [:state_event, :milestone_id, :assignee_id, :label_ids, :add_label_ids, :remove_label_ids].each do |key|
+      %i(state_event milestone_id assignee_id add_label_ids remove_label_ids).each do |key|
         issue_params.delete(key) unless issue_params[key].present?
       end
 

spec/services/issues/bulk_update_service_spec.rb

@@ -148,6 +148,14 @@ describe Issues::BulkUpdateService, services: true do
       it 'does not update issues not passed in' do
         expect(issue_bug_and_regression.label_ids).to contain_exactly(bug.id, regression.id)
       end
+
+      context 'when those label IDs are empty' do
+        let(:labels) { [] }
+
+        it 'updates the issues passed to have no labels' do
+          expect(issues.map(&:reload).map(&:label_ids)).to all(be_empty)
+        end
+      end
     end
 
     context 'when add_label_ids are passed' do