messageTray: Remove transient sources

As far as I can tell, the only behavior change of a transient source
is that they auto-destroy after viewing their summary box pointer.
Since all transient sources are only associated with transient
notifications, it seems that we can never get to their summary box
pointer in the first place! Remove support for this.

https://bugzilla.gnome.org/show_bug.cgi?id=710115