slider: Stop emulating drags in key handling

Emitting ::drag-end after changing the slider value via arrow keys
was a cheap way to make the sound feedback work for keyboard input.
But now that the volume indicator plays the sound on ::value-changed
as well, we can stop doing that - after all, key presses aren't drags.

Besides that, this will make the limiting of feedback to actual volume
changes from the previous commit work for key events as well.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/53

js/ui/slider.js

@@ -181,9 +181,7 @@ var Slider = GObject.registerClass({
         let key = keyPressEvent.keyval;
         if (key == Clutter.KEY_Right || key == Clutter.KEY_Left) {
             let delta = key == Clutter.KEY_Right ? 0.1 : -0.1;
-            this.emit('drag-begin');
             this.value = Math.max(0, Math.min(this._value + delta, this._maxValue));
-            this.emit('drag-end');
             return Clutter.EVENT_STOP;
         }
         return Clutter.EVENT_PROPAGATE;