Bug 1061854 - Dismiss notification swiping up r=mhenretty

With the new ambient indicator, in order to dismiss a notification, a swipe up gesture is needed. All the swipe left code has been removed in this PR.
I'm more a fan of moving things to readable functions instead of using comments, but if that's not the pattern you are following in the project, I'm happy to adapt.
Regarding the second comment, you are right. We shouldn't remove the notification from the tray. The indicator will show as it should, but we are removing the actual notification from the tray. Good catch :)