Simplify bitboards
This PR:
- retires the unused distance() functions in bitboard.h including the 4K SquareDistance LUT,
- makes the 64K PopCnt16 LUT constexpr and only compiles it in if we don't use a popcount intrinsic (we can't fully remove the LUT because it's faster than the intrinsic in the absence of a popcount machine instruction),
- retires Bitboards::init().
Passed STC:
https://tests.stockfishchess.org/tests/view/6a48e6d3f97ff95f78795a8f
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 162016 W: 42054 L: 41976 D: 77986
Ptnml(0-2): 383, 17375, 45454, 17373, 423
I didn't run LTC because the changes only really impact workers without popcount and there aren't any present on fishtest ATM. The correctness of the constexpr PopCnt16 LUT was verified locally with a x86-64 build; the build yields the same signature as master.
closes https://github.com/official-stockfish/Stockfish/pull/6952
No functional change