Remove unused legacy filter init

app/assets/javascripts/init_legacy_filters.js

-/* eslint-disable no-new */
-import LabelsSelect from './labels_select';
-import subscriptionSelect from './subscription_select';
-import UsersSelect from './users_select';
-import issueStatusSelect from './issue_status_select';
-import MilestoneSelect from './milestone_select';
-
-export default () => {
-  new UsersSelect();
-  new LabelsSelect();
-  new MilestoneSelect();
-  issueStatusSelect();
-  subscriptionSelect();
-};