Simplify futilityValue formula
Passed STC non-reg:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 51616 W: 13479 L: 13279 D: 24858
Ptnml(0-2): 134, 5897, 13530, 6129, 118
https://tests.stockfishchess.org/tests/view/6a6524b7c054e285ae028b92
Passed LTC non-reg:
LLR: 2.97 (-2.94,2.94) <-1.75,0.25>
Total: 400380 W: 103781 L: 103944 D: 192655
Ptnml(0-2): 178, 43099, 113790, 42954, 169
https://tests.stockfishchess.org/tests/view/6a6673d4c054e285ae028d93
closes https://github.com/official-stockfish/Stockfish/pull/7017
Bench: 2829394