st: Remove st-tooltip

StTooltip has been plagued by lots of issues, and we recently ditched
it in the dash. Remove it for good.

https://bugzilla.gnome.org/show_bug.cgi?id=670034

js/ui/main.js

@@ -190,7 +190,6 @@ function start() {
     let constraint = new Clutter.BindConstraint({ source: global.stage,
                                                   coordinate: Clutter.BindCoordinate.SIZE });
     uiGroup.add_constraint(constraint);
-    St.set_ui_root(global.stage, uiGroup);
     global.window_group.reparent(uiGroup);
     global.overlay_group.reparent(uiGroup);
     global.stage.add_actor(uiGroup);