Simplify Futility pruning formula
Passed STC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 170112 W: 44266 L: 44193 D: 81653
Ptnml(0-2): 599, 20062, 43611, 20235, 549
https://tests.stockfishchess.org/tests/view/68f50b94637acd2a11e721ad
Passed LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 164658 W: 42393 L: 42318 D: 79947
Ptnml(0-2): 120, 18127, 45747, 18228, 107
https://tests.stockfishchess.org/tests/view/68fa6683637acd2a11e72ac3
closes https://github.com/official-stockfish/Stockfish/pull/6389
bench: 2351426