cpu: Get rid of the (read|set)RegOtherThread methods.

cpu: Get rid of the (read|set)RegOtherThread methods.

These are implemented by MIPS internally now.

Change-Id: If7465e1666e51e1314968efb56a5a814e62ee2d1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18436
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>