[TableGen] Remove & Replace old GICombiner Backend

The MatchTable-based GlobalISel Combiner backend is the new default. There are no in-tree users left of the old backend.

- Removed implementation of old MatchDAG-based Combiner, including tests, the backend itself and all supporting code.
- Renamed MatchTable backend to `GlobalISelCombinerEmitter.cpp` + removed "-matchtable" from its CL option.
    - no need to have a verbose name as it's the only backend left now.

Reviewed By: aemerson

Differential Revision: https://reviews.llvm.org/D158710