Remove unneeded config line for AMS.
Remove unneeded config line for AMS.

The current version of AMS, 0.10.0, no longer requires this config
for rails-api apps.

New docs: https://github.com/rails-api/active_model_serializers/blob/master/docs/general/getting_started.md#rails-integration
Old docs: https://github.com/rails-api/active_model_serializers/tree/0-9-stable#use-serialization-outside-of-actioncontrollerbase

rails-api/rails-api