backends: Remove x/y arguments from MetaCursorTracker::cursor-moved

Make this signal a hint, the actual coordinates should be queried to the
cursor tracker, the device, the seat... There's enough options.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

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

@@ -237,8 +237,6 @@ sync_cursor_state (MetaScreenCastMonitorStreamSrc *monitor_src)
 
 static void
 cursor_moved (MetaCursorTracker              *cursor_tracker,
-              float                           x,
-              float                           y,
               MetaScreenCastMonitorStreamSrc *monitor_src)
 {
   sync_cursor_state (monitor_src);