The systray_cleanup() function gives up the _NET_SYSTEM_TRAY_Sn
selection ownership and should be called only when Awesome exits.
Calling systray_cleanup() from drawin_systray_kickout() is not correct,
because drawin_systray_kickout() is called when the system tray window
needs to be hidden, including the case when the last tray icon was
removed and the tray became empty.
Fixes: https://github.com/awesomeWM/awesome/issues/2301
Signed-off-by: Sergey Vlasov <sigprof@gmail.com>