Remove usage of dismissMultipleVulnerabilities feature flag in FE code

What does this MR do and why?

This change removes the usage of the dismissMultipleVulnerabilities feature flag from the frontend code.

#417883 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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

@@ -10,7 +10,6 @@ import {
   GlLink,
 } from '@gitlab/ui';
 import * as Sentry from '@sentry/browser';
-import glFeatureFlagMixin from '~/vue_shared/mixins/gl_feature_flags_mixin';
 import eventHub from 'ee/security_dashboard/utils/event_hub';
 import { __, s__, n__ } from '~/locale';
 import toast from '~/vue_shared/plugins/global_toast';
@@ -29,7 +28,6 @@ export default {
     GlFormGroup,
     GlLink,
   },
-  mixins: [glFeatureFlagMixin()],
   inject: ['vulnerabilitiesQuery', 'vulnerabilitiesCountsQuery'],
   props: {
     selectedVulnerabilities: {
@@ -176,7 +174,7 @@ export default {
 
       this.isSubmitting = true;
 
-      if (this.isDismissedStatus && this.glFeatures.dismissMultipleVulnerabilities) {
+      if (this.isDismissedStatus) {
         return this.dismissMultipleVulnerabilities();
       }