sim: delete dead current_state globals

sim: delete dead current_state globals

The global current_state handle to the current simulator state is a
design idea that was half implemented, but never really cleaned up.
The point was to have a global variable pointing to the state so that
funcs could more quickly & easily access the state anywhere.  We've
instead moved in the direction of passing state around everywhere and
don't have any intention of moving back.

I also can't find any references to gdb using this variable, or to
cgen related "dump_regs" functions, both of which were used in the
comments related to this code.