media-manager: Don't run deinitialize GStreamer in finalize()

This makes running tests harder as we cannot call gst_init() after gst_deinit()
has been called.

This is what the API reference has to say about it at
https://gstreamer.freedesktop.org/documentation/gstreamer/gst.html?gi-language=c#gst_deinit

It is normally not needed to call this function in a normal application as the
resources will automatically be freed when the program terminates. This function
is therefore mostly used by testsuites and other memory profiling tools.

plugins/sip/calls-sip-media-manager.c

@@ -136,7 +136,6 @@ static void
 calls_sip_media_manager_finalize (GObject *object)
 {
   CallsSipMediaManager *self = CALLS_SIP_MEDIA_MANAGER (object);
-  gst_deinit ();
 
   g_list_free (self->preferred_codecs);
   g_object_unref (self->settings);