Don't call gst_deinit when exiting Clementine.

It sometimes cause a deadlock since we migrated to GStreamer 1.0.
And it seems we shouldn't need to call it as we are "a normal application":
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/gstreamer-Gst.html#gst-deinit

src/engines/gstengine.cpp

@@ -113,9 +113,6 @@ GstEngine::~GstEngine() {
 
   current_pipeline_.reset();
 
-  // Save configuration
-  gst_deinit();
-
   qDeleteAll(device_finders_);
 }