Remove unused constexpr_lsb and lsb_index64
The de Bruijn constexpr LSB helper and its 64-entry lookup table have zero references repo-wide. Runtime lsb() uses compiler intrinsics (__builtin_ctzll / _BitScanForward64), and compile-time table builds no longer need it. Leftover from earlier refactors, same class as PawnPushOrAttacks removal. Verified with repo-wide grep and syntax check on bitboard.cpp / position.cpp / attacks.cpp.
closes https://github.com/official-stockfish/Stockfish/pull/7158
No functional change