arch: Get rid of (some) unused VAddr types.

arch: Get rid of (some) unused VAddr types.

X86 actually defines and uses a VAddr bitunion, but the ARM, MIPS and
SPARC versions are just stubs and aren't used anywhere.

Change-Id: Iea8d0c8ab04ac1d95f49458f0fc41f291751da1a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33202
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>