Remove widget_visibility_polling feature flag

This flag was added in 12.10 to allow instance admins to opt out of
polling behavior.

app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue

@@ -307,8 +307,7 @@ export default {
         callback: this.checkStatus,
         startingInterval: this.startingPollInterval,
         maxInterval: this.startingPollInterval + secondsToMilliseconds(4 * 60),
-        hiddenInterval:
-          window.gon?.features?.widgetVisibilityPolling && secondsToMilliseconds(6 * 60),
+        hiddenInterval: secondsToMilliseconds(6 * 60),
         incrementByFactorOf: 2,
       });
     },

app/controllers/projects/merge_requests_controller.rb

@@ -28,7 +28,6 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   before_action :authenticate_user!, only: [:assign_related_issues]
   before_action :check_user_can_push_to_source_branch!, only: [:rebase]
   before_action only: [:show] do
-    push_frontend_feature_flag(:widget_visibility_polling, @project, default_enabled: true)
     push_frontend_feature_flag(:mr_commit_neighbor_nav, @project, default_enabled: true)
     push_frontend_feature_flag(:multiline_comments, @project, default_enabled: true)
     push_frontend_feature_flag(:file_identifier_hash)

config/feature_flags/development/widget_visibility_polling.yml

----
-name: widget_visibility_polling
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/29318
-rollout_issue_url: 
-milestone: '12.10'
-type: development
-group: group::code review
-default_enabled: true