batman-adv: remove a misleading comment

As much as I'm happy to see LWN links sprinkled through the kernel by the
dozen, this one in particular reflects a very old state of reality; the
associated comment is now incorrect.  So just delete it.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>