Decoder: Remove the thread context get/set from the decoder.

Decoder: Remove the thread context get/set from the decoder.

This interface is no longer used, and getting rid of it simplifies the
decoders and code that sets up the decoders. The thread context had been used
to read architectural state which was used to contextualize the instruction
memory as it came in. That was changed so that the state is now sent to the
decoders to keep locally if/when it changes. That's significantly more
efficient.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>