Remove material imbalance param when adjusting optimism
Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/664d033d830eb9f886616aff
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 102144 W: 26283 L: 26135 D: 49726
Ptnml(0-2): 292, 12201, 25991, 12243, 345
Passed non-regression LTC:
https://tests.stockfishchess.org/tests/view/664d5c00830eb9f886616cb3
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 250032 W: 63022 L: 63036 D: 123974
Ptnml(0-2): 103, 27941, 68970, 27871, 131
closes https://github.com/official-stockfish/Stockfish/pull/5284
Bench: 1330940