Simplify bestvalue update formula
Passed STC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 124960 W: 32598 L: 32472 D: 59890
Ptnml(0-2): 480, 14852, 31694, 14970, 484
https://tests.stockfishchess.org/tests/view/67b348bae00eea114cdba37d
Passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 150306 W: 38220 L: 38132 D: 73954
Ptnml(0-2): 98, 16430, 42005, 16526, 94
https://tests.stockfishchess.org/tests/view/67b5e37918a66624a7a3f751
closes https://github.com/official-stockfish/Stockfish/pull/5898
Bench: 2146010