playbin2: disable streamsynchronizer magic for this release

playbin2: disable streamsynchronizer magic for this release

Some things aren't quite right yet and cause problems (0-sized buffers
with PREROLL flag set cause crashes in elements that don't expect those;
getting pipeline back to preroll/playing again when audio/video streams
have different lengths and a seek past the end of one of the stream
happens doesn't always work, etc.). Needs further investigation in the
next cycle.

https://bugzilla.gnome.org/show_bug.cgi?id=633700
https://bugzilla.gnome.org/show_bug.cgi?id=634699