Simplify the use of classical eval

no benefit of the fallback term (exercised rarely).
Cleanup the associated code.

passed STC
https://tests.stockfishchess.org/tests/view/62f62c2b6f0a08af9f776367
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 67832 W: 18334 L: 18148 D: 31350
Ptnml(0-2): 369, 7171, 18609, 7439, 328

passed LTC
https://tests.stockfishchess.org/tests/view/62f68beb6f0a08af9f77710e
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 104664 W: 28363 L: 28233 D: 48068
Ptnml(0-2): 169, 10162, 31511, 10350, 140

closes https://github.com/official-stockfish/Stockfish/pull/4132

Bench: 6079565