Remove PvNode dimension from Reductions array

This is a functional simplification: if we simply subtract one to Reductions[]
when PvNode is set, we can remove this dimension of the multidimensional array.
I think this saves about 8K of memory.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 10118 W: 2282 L: 2138 D: 5698
http://tests.stockfishchess.org/tests/view/5c6332b60ebc5925cffbdfed

LTC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 70765 W: 11617 L: 11575 D: 47573
http://tests.stockfishchess.org/tests/view/5c63379e0ebc5925cffbe0de

Closes https://github.com/official-stockfish/Stockfish/pull/2010

Bench 3261078