network: Clear connectivity queue after portal login

network: Clear connectivity queue after portal login

When full connectivity (or rather, non-portal connectivity) is achieved,
we can clear all active portal helpers.

Normally, the portal helper will signal that it is done, but it is
implemented in a racy manner. So here let's do a comprehensive big
hammer solution to guarantee the portal helper definitely gets
eventually closed even if we miss the race window in the portal helper.
(The race is: the portal helper checks connectivity status either
immediately or within a certain amount of time after a navigation policy
decision. This could theoretically be too soon, and if missed, it won't
be rescheduled.)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3813>
(cherry picked from commit b8b690eb88db034b606e51cff64741557a72fc86)