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/status/volume.js

@@ -22,7 +22,7 @@ const Indicator = new Lang.Class({
     Extends: PanelMenu.SystemStatusButton,
 
     _init: function() {
-        this.parent('audio-volume-muted', null, _("Volume"));
+        this.parent('audio-volume-muted', _("Volume"));
 
         this._control = new Gvc.MixerControl({ name: 'GNOME Shell Volume Control' });
         this._control.connect('state-changed', Lang.bind(this, this._onControlStateChanged));