Simplify En Passant

Simplify En Passant

simplifies the handling of en passant during search, needs a little more care in initialization.

Passed STC:
LLR: 2.95 (-2.94,2.94) {-1.25,0.25}
Total: 72608 W: 6569 L: 6559 D: 59480
Ptnml(0-2): 233, 5117, 25629, 5057, 268
https://tests.stockfishchess.org/tests/view/600f1363735dd7f0f0352ce7

Passed LTC:
LLR: 2.92 (-2.94,2.94) {-0.75,0.25}
Total: 24328 W: 913 L: 864 D: 22551
Ptnml(0-2): 10, 731, 10633, 780, 10
https://tests.stockfishchess.org/tests/view/600f2e93735dd7f0f0352cf6

closes https://github.com/official-stockfish/Stockfish/pull/3330

No functional change.