arm64: cacheflush: Remove stale comment

Remove stale comment since commit a7ba121215 ("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.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/cacheflush.h

@@ -30,11 +30,6 @@
  *	the implementation assumes non-aliasing VIPT D-cache and (aliasing)
  *	VIPT I-cache.
  *
- *	flush_cache_mm(mm)
- *
- *		Clean and invalidate all user space cache entries
- *		before a change of page tables.
- *
  *	flush_icache_range(start, end)
  *
  *		Ensure coherency between the I-cache and the D-cache in the