powerpc/mm/hash64: Remove comment that is no longer valid

hash_low_64.S was removed in commit a43c0eb8 ("powerpc/mm: Convert
4k insert from asm to C") and flush_hash_page() is no longer called
from any assembly routine.

Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
[mpe: Tweak comment wording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200721091915.205006-1-santosh@fossix.org