Simplify blending eval with nnue complexity

Simplify blending eval with nnue complexity

Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/66567377a86388d5e27dd89c
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 144000 W: 37443 L: 37338 D: 69219
Ptnml(0-2): 587, 17260, 36208, 17351, 594

Passed non-regression LTC:
https://tests.stockfishchess.org/tests/view/66567f29a86388d5e27dd924
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 112326 W: 28550 L: 28421 D: 55355
Ptnml(0-2): 66, 12732, 30434, 12869, 62

closes https://github.com/official-stockfish/Stockfish/pull/5305

bench 1554486