Simplify futility pruning
Passed non regression STC:
https://tests.stockfishchess.org/tests/view/693e642c46f342e1ec20f68d
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 107968 W: 28080 L: 27937 D: 51951
Ptnml(0-2): 381, 12708, 27626, 12925, 344
Passed non regression LTC:
https://tests.stockfishchess.org/tests/view/693ff10c46f342e1ec20fa6a
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 334266 W: 85271 L: 85370 D: 163625
Ptnml(0-2): 179, 36395, 94086, 36292, 181
closes https://github.com/official-stockfish/Stockfish/pull/6484
Bench: 2987379