arm64: mm: Remove unused support for Normal-WT memory type

The Normal-WT memory type is unused, so remove it and reclaim a MAIR.

Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210527110319.22157-4-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>