ScreenCast: Pass PipeWire stream node ID directly

As of commit 5f5ef3de2cdc816dab82cb7eb5d7171bee0ad2c5 in pipewire the
stream creator can find out the node ID of the stream it created.

So instead of using a special purpose entry to the info property box to
let the application discover stream by monitoring added nodes searching
for the given special purpose entry, just pass the node directly.

https://bugzilla.gnome.org/show_bug.cgi?id=784199

src/backends/meta-screen-cast-monitor-stream-src.c

@@ -147,13 +147,9 @@ meta_screen_cast_monitor_stream_src_record_frame (MetaScreenCastStreamSrc *src,
 
 MetaScreenCastMonitorStreamSrc *
 meta_screen_cast_monitor_stream_src_new (MetaScreenCastMonitorStream  *monitor_stream,
-                                         const char                   *stream_id,
                                          GError                      **error)
 {
-  return g_initable_new (META_TYPE_SCREEN_CAST_MONITOR_STREAM_SRC,
-                         NULL,
-                         error,
-                         "stream-id", stream_id,
+  return g_initable_new (META_TYPE_SCREEN_CAST_MONITOR_STREAM_SRC, NULL, error,
                          "stream", monitor_stream,
                          NULL);
 }