Simplify history update.
Removal of the slowdown factor from the history update formula with
corresponding adjustment of the stat bonus used in the search.
Passed STC:
https://tests.stockfishchess.org/tests/view/655e1079136acbc573544744
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 128096 W: 32355 L: 32235 D: 63506
Ptnml(0-2): 466, 15187, 32573, 15405, 417
Passed LTC:
https://tests.stockfishchess.org/tests/view/655f4e60136acbc573546266
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 50652 W: 12653 L: 12459 D: 25540
Ptnml(0-2): 28, 5666, 13751, 5846, 35
closes https://github.com/official-stockfish/Stockfish/pull/4883
Bench: 1303857