Remove unused/unusable unsubscribe since we don't want to unsubscribe

In 0.9 (which this implementation is based on), the instrumentation
was `!serialize.active_model_serializers`.

rails-api#596

The '!' in the event name meant the event wasn't meant for
production.

https://github.com/rails/rails/pull/10446/files#r4075679

Since we intend the event for production and have a log subscriber,
if we unsubscribe from `render.active_model_serializers`, we'll
break other tests that are relying on that event being subscribed.

Author: bf4

lib/active_model_serializers/test/serializer.rb

@@ -69,10 +69,6 @@ def subscribe
           end
         end
 
-        def unsubscribe
-          ActiveSupport::Notifications.unsubscribe(event_name)
-        end
-
         private
 
         def matches_class?