UCT virtual_loss: Revert default change

UCT virtual_loss: Revert default change

It seems all our virtual_loss tuning was bogus due to a mistake
in the option setting code - all tests were in fact done with
virtual_loss zero.

High virtual_loss has catastrophic effect on gameplay - it transforms
the tree to long narrow corridors with high difference between black
and white node values. I'm not 100% sure about the cause even though
I spent quite long time trying to debug this, but I think it's because
the noise introduced by updates lost due to conflicts is just too high.

Moreover, with virtual_loss raising with number of threads, both the
noise rate and noise magnitude goes up, amplifying the effect.