Remove deprected route

Cleanup old route redirect for usage trends.

changelogs/unreleased/321670-remove-deprecated-instance-statistics-route.yml

+---
+title: Remove old redirect rule for the usage trends feature
+merge_request: 60485
+author:
+type: changed

config/routes/admin.rb

@@ -104,7 +104,6 @@ namespace :admin do
 
   resources :projects, only: [:index]
 
-  get '/instance_statistics', to: redirect('admin/usage_trends')
   resources :usage_trends, only: :index
   resource :dev_ops_report, controller: 'dev_ops_report', only: :show
   resources :cohorts, only: :index