ACPI/IORT: Remove temporary iort_get_id_mapping_index() ACPICA guard

In IORT issue C SMMUv3 IORT nodes gained an additional field (DeviceID
mapping index) so that the SMMUv3 can describe its MSI interrupts.

Referring to it in the kernel requires ACPICA changes and in order
to prevent kernel<->ACPICA dependencies kernel code depending on the
SMMUv3 DeviceID mapping index field was guarded with an ACPICA version
conditional.

ACPICA changes introducing DeviceID mapping index in the IORT structs
were integrated in the kernel with:

commit 4c106aa4 ("ACPICA: iasl: Add SMMUv3 device ID mapping index
support")

so the temporary ACPICA guard has become stale and can be removed.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>