NetworkMenu: don't query DBus properties of removed objects

Calling nm_access_point_get_ssid() in the handler of the
access-point-removed signal can result in DBus request, which will
then fail because the object was already removed at the server side.
Instead, use a difference function to retrieve the access point
object (the network), that compares directly by object identity.

https://bugzilla.gnome.org/show_bug.cgi?id=651378