sim: h8300: delete opcode caching

This is in preparation for converting h8300 over to the common memory
framework.  It's not clear how much of a speed gain this was providing
in the first place -- a naive test of ~400k insns (using shlr.s) shows
that this code actually slowed things down a bit.

If anyone really cares about h8300 anymore, they can migrate to the
common insn caching logic.