dev: Scrub out some lingering uses of MemObject.

dev: Scrub out some lingering uses of MemObject.

MemObject doesn't do anything any more, and is basically just an alias
for ClockedObject.

Change-Id: Ic0e1658609e4e1d7f4b829fbc421f222e4869dee
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20719
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>