Revert "mm: add arch hook to validate mmap() prot flags"

This reverts commit cb1a393c.

Since the arm64 WXN patch has been reverted, remove this hook as it
would not have any users.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/ZfGESD3a91lxH367@arm.com