tests/examples/seek/seek.c: Remove obsolete stream_time reset after flushing seek, core does that automatically now.

Original commit message from CVS:
* tests/examples/seek/seek.c: (do_seek),
(rate_spinbutton_changed_cb), (update_streams), (main):
Remove obsolete stream_time reset after flushing seek, core does that
automatically now.
Improve accuracy of speed spinbutton.
Only do playbin2 stuff when we actually use it.