Simplify Declaration on Pawn Move Generation

Simplify Declaration on Pawn Move Generation

Removes possible micro-optimization in favor of readability.

STC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 75432 W: 5824 L: 5777 D: 63831
Ptnml(0-2): 178, 4648, 28036, 4657, 197
https://tests.stockfishchess.org/tests/view/611fa7f84977aa1525c9cb75

LTC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 41200 W: 1156 L: 1106 D: 38938
Ptnml(0-2): 13, 981, 18562, 1031, 13
https://tests.stockfishchess.org/tests/view/611fcc694977aa1525c9cb9b

Closes https://github.com/official-stockfish/Stockfish/pull/3669

No functional change