messageTray: Don't set _notificationRemoved if destroyed notification is queued

When the user clears all notifications from the notification menu UI, it's
possible for a queued notification to be destroyed after the currently displayed
notification. The removal of the currently displayed notification is not
processed until the notification menu is closed (due to `this._bannerBlocked`).
By then, it's possible that `_notificationRemoved` has already been overwritten
when `_onNotificationDestroy` is invoked with another (queued) notification.

This eventually results in a notification banner that cannot be removed by the
user as the notification object needed to do so has already been destroyed.

Fix this by only assigning to `_notificationRemoved` if `this._notification ==
notification`.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2736>
(cherry picked from commit cde7d44a)