@Persistent should not be considered when scanning for MongoDB entities

[wilkinsona]

As a result of spring-projects/spring-data-mongodb#3592, Spring Data MongoDB has stopped looking for @Persistent when scanning for entities. We should align our behaviour.

A similar change was made in Spring Data Cassandra (spring-projects/spring-data-cassandra#1100) but we've already picked up that change through our use of org.springframework.data.cassandra.config.CassandraEntityClassScanner.