remove ELO levels
It sucks anyway. User can use node and or depth limits instead.
No post futility pruning when skip_null (3514595)
This can happen in two scenarios:
1/ parent node is doing a null search: in this case, we want to avoid the double null move
effect at the horizon.
2/ parent node is doing an IID search: here, we want to return a best move, so pruning early
is actually wasteful.