arm64: Remove pointless WARN_ON in DMA teardown

We expect arch_teardown_dma_ops() to be called very late in a device's
life, after it has been removed from its bus, and thus after the IOMMU
bus notifier has run. As such, even if this funny little check did make
sense, it's unlikely to achieve what it thinks it's trying to do anyway.
It's a residual trace of an earlier implementation which didn't belong
here from the start; belatedly snuff it out.

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