Suppress compile error in unimplemented method

Still haven't implemented vertex-centric BP methods.  Fix a bit of
dead code in one of the unimplemented methods that caused compilation
failure.
Remove last use of KeyNot in graph query builder

Changed KeyNot.of(KeyAtom.of(value != null)) to KeyAtom.of(value ==
null).  This is only permissible in the case where the underlying
TitanKey already exists.  If the key does not exist, the standard
element querying answering code will return the wrong result; this
case is handled separately.
Cleanup refactoring in TitanGraphQueryBuilder
Comment changes in StandardTitanTx