Simplify thread selection
The depth dependency in best thread selection no longer seems to gain sufficiently and so can be simplified away.
Passed non-reg STC SMP:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 437904 W: 111841 L: 112051 D: 214012
Ptnml(0-2): 485, 49454, 119332, 49148, 533
https://tests.stockfishchess.org/tests/view/6a3158590d5d4b19d08055cd
Passed non-reg LTC SMP:
LLR: 3.11 (-2.94,2.94) <-1.75,0.25>
Total: 484778 W: 123864 L: 124093 D: 236821
Ptnml(0-2): 121, 49237, 143920, 48972, 139
https://tests.stockfishchess.org/tests/view/6a377f3ba4f63d3271af0920
closes https://github.com/official-stockfish/Stockfish/pull/6935
No functional change