Remove unreachable code in PersistenceBuilder

Many-type associations are skipped earlier in the function, so these blocks are no longer reached.
OC refactoring for PersistentCollection

This removes an assignment of isDirty to true after iterating through "NEW objects"; however, since $newObjects would only be populated if the collection was dirty (per the earlier if condition), the assignment was likely redundant.
Merge pull request #931 from doctrine/pb-cleanup

PersistentCollection and PersistenceBuilder cleanup