Reduce lock contention in idle_loop

Release split point lock before to wake up
master thread. This seems to increase speed
in case "sleeping threads" are used:

After 7792 games with 4 threads at very fast TC (2"+0.05)
Mod vs Orig 1722 - 1627 - 4443 ELO +4 (+- 5.1)

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/thread.h

@@ -40,7 +40,6 @@ struct SplitPoint {
   Depth depth;
   Value beta;
   int nodeType;
-  int ply;
   int master;
   Move threatMove;