Remove App = ProcessModel class "alias"

- This was put in place in 8c1fdb7
  during the App->ProcessModel rename refactor to address the situation
  where there may have been outstanding DelayedJob jobs that were
  referring to the old class name
- It's been over a year since this work was completed and there have
  been many releases since (this change shipped in capi-release 1.37.0)
  so it should be safe to remove this "alias"

[138382103]

app/models/runtime/process_model.rb

@@ -607,7 +607,5 @@ def logger
       end
     end
   end
-
-  App = ProcessModel
   # rubocop:enable ClassLength
 end

spec/unit/models/runtime/process_model_spec.rb

@@ -1732,12 +1732,5 @@ def act_as_cf_admin
         end
       end
     end
-
-    describe 'App alias' do
-      it_should_be_removed(
-        by: '2018/07/05',
-        explanation: 'We renamed App to ProcessModel in July 2017. It should be safe to remove the alias now without breaking DelayedJob jobs during rolling deploys'
-      )
-    end
   end
 end