Symmetrize optimism
Removes some additional parameters, making the term more logical at the same
time.
Passed STC:
https://tests.stockfishchess.org/tests/view/6550e896136acbc5735328ed
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 271104 W: 68441 L: 68480 D: 134183
Ptnml(0-2): 827, 32590, 68816, 32433, 886
Passed LTC:
https://tests.stockfishchess.org/tests/view/65523858136acbc5735344f7
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 198954 W: 49250 L: 49211 D: 100493
Ptnml(0-2): 93, 22565, 54117, 22614, 88
closes https://github.com/official-stockfish/Stockfish/pull/4874
Bench: 1334248