Remove submodule (fixing invalid initialization)
Correctly adding [coredata-threadsafe] submodule.

As its name implies, this project provides a thread-safe extension to
the iOS Core Data framework. It is drop-in compatible with existing
Core Data projects (insomuch as is practical), requiring only that
references to NSManagedObject and NSManagedObjectContext be replaced
with references to their thread-safe counterparts.