Remove threading code from dicom database and indexer

See discussion at #292.

Pending a re-implementation, remove thread support from the
ctkDICOMDatabase code.  Since the ctkDICOMDatabase internally
manages the database connection, the threading code should make
multiple instances of the database (one per thread) rather than
calling the same instance from multiple threads.  Because of this
there is no reason to have a mutex internal to the class.
Related call, methods, and member variables have been removed.