arch-arm: Remove deadcode from AArch64 address translation

arch-arm: Remove deadcode from AArch64 address translation

There's no need to check for CPSR.WIDTH: if the 64 bit version
of the AT instruction/register is used, it means we are already
in AArch64 execution mode

Change-Id: I1263dcfd04e791eb390199546c177a926c71c6d5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33355
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>