mm, arc: remove obsolete pagefault oom killer comment

Commit 609838cf ("mm: invoke oom-killer from remaining unconverted page
fault handlers") converted arc to call pagefault_out_of_memory(), so remove
the comment about future conversion.

Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>