Staging: batman-adv: Remove batman-adv from staging

batman-adv is now moved to net/batman-adv/ and can be removed from
staging.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>