arm64/dma-mapping: Remove default domain workaround

With the IOMMU core now taking care of default domains for groups
regardless of bus type, we can gleefully rip out this stop-gap, as
slight recompense for having to expand the other one.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>