Disable overview in classic session

The overview is one of the defining features of GNOME 3, and thus almost by definition at odds with the classic session, which emulates a traditional GNOME 2 desktop.

Even with the less prominent placement inside the application menu it never quite fit in - it doesn't help that besides the different UI paradigma, the overview keeps its "normal" styling which differs greatly with classic's normal mode.

So besides removing the "Activities" button via the session mode definition, now that the apps-menu extension doesn't replace it anymore, disable the overview completely in the classic session.