Simplify cutnode reduction formula
Passed STC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 137994 W: 34705 L: 34603 D: 68686
Ptnml(0-2): 124, 15371, 37903, 15477, 122
https://tests.stockfishchess.org/tests/view/66aeb74b4ff211be9d4eda10
Passed LTC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 131456 W: 34148 L: 34031 D: 63277
Ptnml(0-2): 506, 15571, 33465, 15672, 514
https://tests.stockfishchess.org/tests/view/66ae258b4ff211be9d4ed95d
closes https://github.com/official-stockfish/Stockfish/pull/5531
Bench: 1261995