Simplify reduced depth search

Simplify reduced depth search

Simplification in reduced depth search.

STC https://tests.stockfishchess.org/tests/view/5f64c72fbb0cae038ca8f531
LLR: 2.94 (-2.94,2.94) {-1.25,0.25}
Total: 28320 W: 3475 L: 3359 D: 21486
Ptnml(0-2): 170, 2485, 8773, 2523, 209

LTC https://tests.stockfishchess.org/tests/view/5f650cfabb0cae038ca8f585
LLR: 2.95 (-2.94,2.94) {-0.75,0.25}
Total: 58392 W: 3354 L: 3285 D: 51753
Ptnml(0-2): 74, 2826, 23336, 2877, 83

closes https://github.com/official-stockfish/Stockfish/pull/3139

bench: 4201295