Simplify TT PV reduction
This also removes some incorrect fail-high logic.
Passed STC:
https://tests.stockfishchess.org/tests/view/65cb3b641d8e83c78bfd04a9
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 87968 W: 22634 L: 22468 D: 42866
Ptnml(0-2): 315, 10436, 22323, 10588, 322
Passed LTC:
https://tests.stockfishchess.org/tests/view/65cccee21d8e83c78bfd222c
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 70794 W: 17846 L: 17672 D: 35276
Ptnml(0-2): 44, 7980, 19189, 8126, 58
closes https://github.com/official-stockfish/Stockfish/pull/5055
Bench: 1474424