Additional cleanup after `expose_dismissal_reason` feature flag

ee/app/assets/javascripts/security_dashboard/components/shared/vulnerability_report/vulnerability_filters.vue

 <script>
 import { debounce, cloneDeep, isEqual } from 'lodash';
-import glFeatureFlagsMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';
 import ActivityFilter from '../filters/activity_filter.vue';
 import ProjectFilter from '../filters/project_filter.vue';
 import ClusterFilter from '../filters/cluster_filter.vue';
@@ -12,7 +11,6 @@ import StatusWithDismissalFilter from '../filters/status_with_dismissal_filter.v
 import { FILTERS } from './constants';
 export default {
-  mixins: [glFeatureFlagsMixin()],
   props: {
     filters: {
       type: Array,