arm64: mte: Drop arch_enable_tagging()

arch_enable_tagging() was left in memory.h after the introduction of
async mode to not break the bysectability of the KASAN KUNIT tests.

Remove the function now that KASAN has been fully converted.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>