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.cpp

@@ -319,7 +319,6 @@ Value ThreadsManager::split(Position& pos, Stack* ss, Value alpha, Value beta,
   sp->master = master;
   sp->cutoff = false;
   sp->slavesMask = 1ULL << master->idx;
-  sp->allSlavesRunning = true;
   sp->depth = depth;
   sp->bestMove = *bestMove;
   sp->threatMove = threatMove;