Set this._notificationRemoved to false in _hideNotificationComplete()

Set this._notificationRemoved to false in _hideNotificationComplete()

We used to set this._notificationRemoved to false in _hideNotification().
However, the user focusing on the associated application can result in the
source being removed while the notification is hiding, in which case
this._notificationRemoved was set to true and never unset. This resulted in
the next notification only showing up briefly and hiding. Moving setting
this._notificationRemoved to false to _hideNotificationComplete() fixes this
behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=611122