arch: Get rid of the unused (and mostly undefined) zeroRegisters.

arch: Get rid of the unused (and mostly undefined) zeroRegisters.

Change-Id: Iadf56e4e742506af7ae4b617d2dc5a56439aa407
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24188
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>