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>

src/arch/SConscript

@@ -61,7 +61,6 @@ env.SwitchingHeaders(
         isa.hh
         isa_traits.hh
         locked_mem.hh
-        microcode_rom.hh
         pseudo_inst.hh
         registers.hh
         remote_gdb.hh