batman-adv: clear ADD+DEL (and viceversa) events in the same orig-interval

During an OGM-interval (time between two different OGM sendings) the same client
could roam away and then roam back to us. In this case the node would add two
events to the events list (that is going to be sent appended to the next OGM). A
DEL one and an ADD one. Obviously they will only increase the overhead (either in
the air and on the receiver side) and eventually trigger wrong states/events
without producing any real effect.

For this reason we can safely delete any ADD event with its related DEL one.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>