DATAJPA-460 - Support query creation for deleteBy / removeBy prefix.

Added implementation of deleteBy / removeBy support for JPA backed repositories. We delete entities by looking them up with the appropriate query and delete them afterwards via entityManager.remove(...). This is rather inefficient but provides the benefit of being able to use the query derivation mechanism for entity deletion as well.

Original pull request: #66.
DATAJPA-460 - Reduce implementation to core requested feature.

Removed the additional deleted flag in @Query as we currently already ship with a method to manually implement delete-queries (using @Modifying and a manually defined JPQL query).

Tiny optimization in DeleteExecution.

Original pull request: #66.