messageTray: Remove deprecated code

messageTray: Remove deprecated code

Originally the Source method for showing notifications was "notify",
which had to change when turning it into a GObject subclass to not
clash with g_object_notify().

That change happened during the 3.36 cycle, so extensions have had
two releases (a year) to adapt to the replacement. That seems long
enough, so remove the deprecated compat code.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3848

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1749>