power: Move the Power Off indicator to the power menu

It's only supposed to show if we have a battery, and hooking into
the power system is the easiest way of making that happen.

https://bugzilla.gnome.org/show_bug.cgi?id=705845

js/ui/status/system.js

@@ -47,8 +47,6 @@ const Indicator = new Lang.Class({
     _init: function() {
         this.parent();
 
-        this._indicator = this.addIndicator(new Gio.ThemedIcon({ name: 'system-shutdown-symbolic' }));
-
         this._screenSaverSettings = new Gio.Settings({ schema: SCREENSAVER_SCHEMA });
         this._lockdownSettings = new Gio.Settings({ schema: LOCKDOWN_SCHEMA });
         this._privacySettings = new Gio.Settings({ schema: PRIVACY_SCHEMA });