arch-arm: Remove m5ops_base declaration from ArmSystem

arch-arm: Remove m5ops_base declaration from ArmSystem

This declaration should have been removed but was accidentally re-added.
It keeps m5ops_base from being passed correctly from Python to C++ when
using ARM ISA, and hence triggers gem5 crash when the guest tries to
call m5ops. This change removes it again to fix the crash.

JIRA: https://gem5.atlassian.net/browse/GEM5-658

Change-Id: I8df4ff19ecc0d64255f24dc991f71b065d2a894e
Signed-off-by: Hsuan Hsu <hsuan.hsu@mediatek.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30914
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>