batman-adv: Remove old fragmentation code

Remove the existing fragmentation code before adding the new version
and delete unicast.{h,c}.

batadv_unicast_send_skb() is moved to send.c and renamed to
batadv_send_skb_unicast().

fragmentation entry in sysfs (bat_priv->fragmentation) is kept for use in
the new fragmentation code.

BATADV_UNICAST_FRAG packet type is renamed to BATADV_FRAG for use in the
new fragmentation code.

Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>