Remove code that is not used anywhere

closes https://github.com/official-stockfish/Stockfish/pull/6201

No functional change

src/bitboard.h

@@ -179,11 +179,6 @@ inline Bitboard between_bb(Square s1, Square s2) {
     return BetweenBB[s1][s2];
 }
 
-// Returns true if the squares s1, s2 and s3 are aligned either on a
-// straight or on a diagonal line.
-inline bool aligned(Square s1, Square s2, Square s3) { return line_bb(s1, s2) & s3; }
-
-
 // distance() functions return the distance between x and y, defined as the
 // number of steps for a king in x to reach y.