Don't store stand pat return value in TT
Passed non-regression STC:
https://tests.stockfishchess.org/tests/view/69d2d3de61a12cebe17ede38
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 68384 W: 17646 L: 17459 D: 33279
Ptnml(0-2): 200, 8058, 17496, 8231, 207
Passed non-regression LTC:
https://tests.stockfishchess.org/tests/view/69d4293433584dad27b3c8cb
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 37290 W: 9582 L: 9380 D: 18328
Ptnml(0-2): 18, 3915, 10579, 4113, 20
closes https://github.com/official-stockfish/Stockfish/pull/6711
Bench: 2923401