This way, clicking a message tray icon while the overview is open will
close the overview when activating its window.
Remove some other overview-related activation code which is now
redundant.
Also, remove calls to "global.get_current_time()" when calling
Main.activateWindow, since it's unnecessary (activateWindow will call
it itself if you don't pass in that arg).
https://bugzilla.gnome.org/show_bug.cgi?id=609765