Remove policy_list.vue component

Summary

with the removal of the :security_orchestration_policies_configuration feature flag, we can remove the policy_list.vue component, which was replaced by policies_list.vue component because there were too many changes that were needed

Improvements

By deleting this code, we will remove dead code from the codebase, which can be confusing for other developers

Risks

If done before the feature flag is removed, it will prevent us from rolling the feature flag back in the event of an issue

Involved components

• frontend delete https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/assets/javascripts/threat_monitoring/components/policy_list.vue and https://gitlab.com/gitlab-org/gitlab/blob/master/ee/spec/frontend/threat_monitoring/components/policy_list_spec.js