calendar: Don't allow event messages to be closed

The built-in calendar isn't meant to replace a full-fledged calendar
app, which is why clearing event messages only hides the event in
gnome-shell rather than deleting the actual event. This has turned out
to not be overly useful and often confusing - it creates a discrepancy
with visible events in apps, isn't revertible in a non-obscure fashion
and non-obviously limited to the current date.

As we are considering moving events out of the message list and back to
the calendar, it looks like a good time to remove that ability and keep
notifications as the only removable messages.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/262

js/ui/messageList.js

@@ -498,7 +498,7 @@ var Message = new Lang.Class({
     },
 
     canClose() {
-        return this._mediaControls.get_n_children() == 0;
+        return false;
     },
 
     _sync() {