arm64/mm: Drop PMD_SECT_VALID

arm64/mm: Drop PMD_SECT_VALID

This just drops off the macro PMD_SECT_VALID which remains unused. Because
macro PMD_TYPE_SECT with same value (_AT(pmdval_t, 1) << 0), gets used for
creating or updating given block mappings.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20240724044712.602210-1-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>