Micro-optimization for SEE: remove a superfluous condition

[pb00068]

The removed condition can never be true, it's superfluous.
It never triggers even with a bench 16 1 20 run.
To met the condition it would imply that the previous recapture was done by a higher rated piece than a Queen.
This is only the case when the King recaptures and that's already handled in line 1161: (return (attackers & ~pieces(stm)) ? res ^ 1).
To maintain the code robust and understandable I added an assert and a comment.
Thanks to mstembera in helping me for this PR.

no functional change
bench: 1767398

[github-actions]

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(execution 13074986246 / attempt 1)

[mstembera]

Nice find! BTW I thought to ask if this works even if one side has multiple queens correct?

[pb00068]

Nice find! BTW I thought to ask if this works even if one side has multiple queens correct?

I think in a 'bench 16 1 20' there should be enough of this kind of positions.
If you have doubts, you can add such a position in benchmark.cpp and verify if the bench changes.
Anyhow I'm pretty sure its completely irrelevant if one side has multiple queens.