batman-adv: mcast: remove now redundant single ucast forwarding

The multicast code to send a multicast packet via multiple batman-adv
unicast packets is not only capable of sending to multiple but also to a
single node. Therefore we can safely remove the old, specialized, now
redundant multicast-to-single-unicast code.

The only functional change of this simplification is that the edge case
of allowing a multicast packet with an unsnoopable destination address
(224.0.0.0/24 or ff02::1) where only a single node has signaled interest
in it via the batman-adv want-all-unsnoopables multicast flag is now
transmitted via a batman-adv broadcast instead of a batman-adv unicast
packet. Maintaining this edge case feature does not seem worth the extra
lines of code and people should just not expect to be able to snoop and
optimize such unsnoopable multicast addresses when bridges are involved.

While at it also renaming a few items in the batadv_forw_mode enum to
prepare for the new batman-adv multicast packet type.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>