[notifications] Clear notification from show queue if it's closed before it's shown

It can happen that a notification is placed in the show queue and then
it's closed (eg. programatically) before it was even shown. In this case
the notification must be cleared from the show queue otherwise it will
get displayed and hidden but the popup is never freed for reuse,
resulting in notificaions starting at higher position from the panel.

BUG: 342605