Remove Threat Extension
Gret code simplification:
- instead do not futility prune threat refutations. allows_move() is therefore removed.
- also remove the !PvNode conditions from SEE pruning (search and qsearch)

4000 games at 50,000 nodes/move:
1085-989-1926 [51.2%] LOS=98.3%

4000 games in 10"+0.1"
756-751-2493 [50.1%] LOS=55.1%

signature = 4343046