Remove the component: mr_widget_jira_association_missing from the MR Widget

The following discussion from !154533 (merged) should be addressed:

• @iamphill started a discussion:

  Can we also remove this from https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js#L5? 😄

  I also think we can remove the component as well (https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/vue_merge_request_widget/components/states/mr_widget_jira_association_missing.vue) but that might need a bit more wok to remove it from everywhere so I can handle this outside of this merge request if you would prefer?