systemtray: Avoid dbus calls after exit
StatusNotifierItemHost has the lifespan of the application. If we have the lifespan of the application calling unregister isn't achieving anything useful.
It's problematic to do DBus calls after the event loop has stop.
See https://errors-eval.kde.org/organizations/kde/issues/178/
BUG: 460814 FIXED-IN: 5.26.2
Edited by Fushan Wen