Simplify worsening deduction in futility margin
Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/66817d46442423e547141226
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 345408 W: 89146 L: 89266 D: 166996
Ptnml(0-2): 954, 41317, 88286, 41189, 958
Passed non-regression LTC:
https://tests.stockfishchess.org/tests/view/66818dbe1e90a146232d1f62
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 173214 W: 43821 L: 43755 D: 85638
Ptnml(0-2): 108, 19407, 47492, 19511, 89
closes https://github.com/official-stockfish/Stockfish/pull/5424
bench 981017