Simplify move history reduction
Recent VLTC search tuning has suggested that the depth limit can be increased
by a lot. This patch simplifies away the depth-based bonus from statScore
reduction, making the divisor a constant.
Passed STC:
https://tests.stockfishchess.org/tests/view/656201f5136acbc573549791
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 91520 W: 23130 L: 22967 D: 45423
Ptnml(0-2): 282, 10947, 23141, 11106, 284
Passed LTC:
https://tests.stockfishchess.org/tests/view/6562b43a136acbc57354a581
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 352902 W: 86796 L: 86917 D: 179189
Ptnml(0-2): 190, 40227, 95741, 40100, 193
closes https://github.com/official-stockfish/Stockfish/pull/4886
Bench: 1297179