mac80211: Unify mesh and mpp path removal function

mpp_path_del() and mesh_path_del() are mostly the same function.
Move common code into a new static function.

Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Henning Rogge <henning.rogge@fkie.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>