gnu: spice-gtk: Only use "good" GStreamer plugins out of the box.

Do not propagate plugins that are known to be patent encumbered out of the
box, so the user has a choice.

* gnu/packages/spice.scm (spice-gtk)[propagated-inputs]: Remove gst-libav,
gst-plugins-bad and gst-plugins-ugly.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>