Remove API changes for assigning group runner to project

Since we are going to handle the API stuff later we can just leave this as error case since the model validations will stop this

lib/api/runners.rb

@@ -205,7 +205,6 @@ module API
       def authenticate_enable_runner!(runner)
         forbidden!("Runner is shared") if runner.is_shared?
         forbidden!("Runner is locked") if runner.locked?
-        forbidden!("Runner is a group runner") if runner.assigned_to_group?
         return if current_user.admin?
 
         forbidden!("No access granted") unless user_can_access_runner?(runner)