Simplify doDeeperSearch formula
Passed STC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 211776 W: 54939 L: 54911 D: 101926
Ptnml(0-2): 714, 24971, 54484, 25011, 708
https://tests.stockfishchess.org/tests/view/6938971875b70713ef796b70
Passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 216774 W: 55346 L: 55326 D: 106102
Ptnml(0-2): 105, 23599, 60980, 23577, 126
https://tests.stockfishchess.org/tests/view/693fc91f46f342e1ec20f9f6
closes https://github.com/official-stockfish/Stockfish/pull/6486
Bench: 3267755