xfrm: remove irrelevant comment in xfrm_input().

This patch removes a comment in xfrm_input() which became irrelevant
due to commit 2774c131, "xfrm: Handle blackhole route creation via afinfo".
That commit removed returning -EREMOTE in the xfrm_lookup() method when the
packet should be discarded  and also removed the correspoinding -EREMOTE
handlers. This was replaced by calling the make_blackhole() method. Therefore
the comment about -EREMOTE is not relevant anymore.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>