powerpc/mm/hash64: Remove comment that is no longer valid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

arch/powerpc/mm/book3s64/hash_utils.c

@@ -1707,10 +1707,6 @@ unsigned long pte_get_hash_gslot(unsigned long vpn, unsigned long shift,
 	return gslot;
 }
 
-/*
- * WARNING: This is called from hash_low_64.S, if you change this prototype,
- *          do not forget to update the assembly call site !
- */
 void flush_hash_page(unsigned long vpn, real_pte_t pte, int psize, int ssize,
 		     unsigned long flags)
 {