Simplify pawn count in evaluation
This simplifies the evaluation by removing the unnecessary pawn count term when
combining nnue and optimism values.
Passed STC
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 61472 W: 15748 L: 15554 D: 30170
Ptnml(0-2): 191, 7123, 15933, 7279, 210
https://tests.stockfishchess.org/tests/view/650c34cf7ca0d3f7bbf264ff
Passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 81264 W: 20657 L: 20500 D: 40107
Ptnml(0-2): 30, 8713, 22997, 8854, 38
https://tests.stockfishchess.org/tests/view/650cc30efb151d43ae6d5987
closes https://github.com/official-stockfish/Stockfish/pull/4800
Bench: 1530568