KVM: arm64: nv: Drop sanitised_sys_reg() helper

Now that we have the infrastructure to enforce a sanitised register
value depending on the VM configuration, drop the helper that only
used the architectural RES0 value.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240214131827.2856277-8-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>