notification: Use the same Source for showing system notifications

This drops all subclasses of `MessageTray.Source` that were used to
display system notifications. Now the `Source` returned from
`MessageTray.getSystemSource()` is used.
Ensure also that properties and methods that where set on the `Source`
are moved to the `Notification` object itself.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3156>

js/ui/messageTray.js

@@ -1348,17 +1348,3 @@ export function getSystemSource() {
 
     return systemNotificationSource;
 }
-
-export const SystemNotificationSource = GObject.registerClass(
-class SystemNotificationSource extends Source {
-    constructor() {
-        super({
-            title: _('System Information'),
-            iconName: 'dialog-information-symbolic',
-        });
-    }
-
-    open() {
-        this.destroy();
-    }
-});