Fix API DoctrineORMSerializationType to be loaded only when using ORM

I've moved `api_form.xml` that only contains form types definitions for API using `DoctrineORMSerializationType` when using the Doctrine ORM context.

This is to avoid the same issue encountered in `SonataUserBundle` here: https://github.com/sonata-project/SonataUserBundle/commit/9027604673cec6b67b5edfb47cba3ae55fd772e7#commitcomment-5705945