Remove dead mentoring requests tracks endpoint (#8345)

The `tracks` action on `RequestsController` called
`Mentor::Request::RetrieveTracks` which was deleted in 2021.
The frontend uses `TracksController#mentored` instead.

Fixes #8324

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: iHiD

app/controllers/api/mentoring/requests_controller.rb

@@ -12,10 +12,6 @@ def index
     render json: AssembleMentorRequests.(current_user, params)
   end
 
-  def tracks
-    render json: Mentor::Request::RetrieveTracks.(current_user)
-  end
-
   def exercises
     render json: Mentor::Request::RetrieveExercises.(current_user, params[:track_slug])
   end

config/routes/api.rb

@@ -216,7 +216,6 @@
 
       resources :requests, only: %i[index], param: :uuid do
         collection do
-          get :tracks
           get :exercises
         end
         member do