Compute SplitPoint::spLevel on the fly

And retire a redundant field. This is important also
from a concept point of view becuase we want to keep
SMP structures as simple as possible with the only
strictly necessary data.

Verified with

dbg_hit_on(sp->spLevel != level)

that the values are 100% the same out of more 50K samples.

No functional change.

src/thread.h

@@ -73,7 +73,6 @@ struct SplitPoint {
   const Position* pos;
   Search::Stack* ss;
   Thread* masterThread;
-  int spLevel;
   Depth depth;
   Value beta;
   int nodeType;