arm: Remove "using namespace ArmISA" from arch/arm/isa_traits.hh.

arm: Remove "using namespace ArmISA" from arch/arm/isa_traits.hh.

This has been in this file since it was created in 2009. No global "using
namespace ${NAMESPACE}" should ever appear in a .hh file since then that
namespace is "used" in all files that include the .hh, even if they
aren't aware of it or even actively don't want to.

Change-Id: Idb7d7c5b959077eb4905fbb2044aa55959b8f37f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34155
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>