arch,cpu,sim: Get rid of the microcode ROM stub code.

arch,cpu,sim: Get rid of the microcode ROM stub code.

This code, including a switching header file, is no longer necessary
because ROM based microops are now handled by the decoder itself.

Change-Id: Ie3ea4a7371dec22993ede80e2acd1df7cd1ecf59
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32899
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>