Remove metrics dashboard alert functionality

Changelog: removed

app/controllers/projects/alert_management_controller.rb

@@ -4,7 +4,7 @@ class Projects::AlertManagementController < Projects::ApplicationController
   before_action :authorize_read_alert_management_alert!
 
   before_action(only: [:index]) do
-    push_frontend_feature_flag(:managed_alerts_deprecation, @project)
+    push_frontend_feature_flag(:managed_alerts_deprecation, @project, default_enabled: :yaml)
   end
 
   feature_category :incident_management

app/controllers/projects/metrics_dashboard_controller.rb

@@ -12,7 +12,7 @@ module Projects
     before_action do
       push_frontend_feature_flag(:prometheus_computed_alerts)
       push_frontend_feature_flag(:disable_metric_dashboard_refresh_rate)
-      push_frontend_feature_flag(:managed_alerts_deprecation, @project)
+      push_frontend_feature_flag(:managed_alerts_deprecation, @project, default_enabled: :yaml)
     end
 
     feature_category :metrics

config/feature_flags/development/managed_alerts_deprecation.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/331863
 milestone: '14.0'
 type: development
 group: group::monitor
-default_enabled: false
+default_enabled: true