bus: Do not override source->prepare

Since GLib 2.36 we do not need it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/718>

gst/gstbus.c

@@ -804,13 +804,6 @@ typedef struct
   GstBus *bus;
 } GstBusSource;
 
-static gboolean
-gst_bus_source_prepare (GSource * source, gint * timeout)
-{
-  *timeout = -1;
-  return FALSE;
-}
-
 static gboolean
 gst_bus_source_check (GSource * source)
 {
@@ -902,7 +895,7 @@ gst_bus_source_finalize (GSource * source)
 }
 
 static GSourceFuncs gst_bus_source_funcs = {
-  gst_bus_source_prepare,
+  NULL,
   gst_bus_source_check,
   gst_bus_source_dispatch,
   gst_bus_source_finalize