Remove stale comment since commit a7ba1212 ("arm64: use asm-generic/cacheflush.h")

Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Link: https://lore.kernel.org/r/1611575753-36435-1-git-send-email-zhangshaokun@hisilicon.comSigned-off-by: Will Deacon <will@kernel.org>