autorunManager: Do not mark the notification CRITICAL

There is nothing particularly critical about this notification, it
was only marked as such to get certain behavior like auto-expanding
and sticking-around to be acknowledged by the user (as it offers
more actions than the summary notification, so it is frustrating
when it goes away because it was missed).
As all notifications will now stay visible until we are sure the
users has seen them, the latter reasoning no longer applies.
Auto-expansion doesn't appear too important and may even be considered
annoying by users, so remove the CRITICAL hint now.

https://bugzilla.gnome.org/show_bug.cgi?id=657923