Simplify ttMove reduction formula
Passed STC non-reg:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 110720 W: 28313 L: 28180 D: 54227
Ptnml(0-2): 230, 12259, 30279, 12332, 260
https://tests.stockfishchess.org/tests/view/69fa69c83a3c3e525bb2b645
Passed LTC non-reg:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 307728 W: 77955 L: 78027 D: 151746
Ptnml(0-2): 138, 33481, 86698, 33409, 138
https://tests.stockfishchess.org/tests/view/6a00a2549392f0c317214085
closes https://github.com/official-stockfish/Stockfish/pull/6826
Bench: 2559757