ACPI/IORT: Remove leftover ACPI_IORT_SMMU_V3_PXM_VALID guard

The conditional ACPI_IORT_SMMU_V3_PXM_VALID guard around
arm_smmu_v3_set_proximity() was added to manage a cross tree
ACPICA merge dependency; with ACPICA changes merged in:

commit c9442300 ("ACPICA: iasl: Update to IORT SMMUv3
disassembling")

the guard has become useless. Remove it.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>