Retire "Active reparenting"

After 6K games at 60" + 0.1 on QUAD with 4 threads
this implementation fails to show a measurable increase,
result is well within error bar.

Perhaps with 8 or more threads resut is better but we
don't have the hardware to test. So retire for now and
in case re-add in the future if it proves good on big
machines.

The only good news is that we don't have a regression and
implementation is stable and bug-free, so could be reused
somewhere in the future.

No functional change.

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

src/thread.h

@@ -57,7 +57,6 @@ struct SplitPoint {
   volatile Move bestMove;
   volatile int moveCount;
   volatile bool cutoff;
-  volatile bool allSlavesRunning;
 };