arch,sim: Stop decoding the pseudo inst subfunc value.

arch,sim: Stop decoding the pseudo inst subfunc value.

This isn't used by anything any more. The func field is left in place
to ensure compatability, but there's no reason to decode a value
nobody is going to use.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I85fcd0e4a362551c29af6bff350d99af86050415
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23179
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>