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
a8005e3c30