Simplify PvNode Reduction

Simplify PvNode Reduction

Remove the depth condition for PvNode reduction.

Simplification STC:
https://tests.stockfishchess.org/tests/view/64d308fa5b17f7c21c0e0303
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 38976 W: 10106 L: 9889 D: 18981
Ptnml(0-2): 129, 4479, 10040, 4726, 114

Simplification LTC:
https://tests.stockfishchess.org/tests/view/64d457db5b17f7c21c0e236f
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 156402 W: 39727 L: 39645 D: 77030
Ptnml(0-2): 71, 17143, 43696, 17215, 76

closes https://github.com/official-stockfish/Stockfish/pull/4747

Bench: 1493904