Replace clear_bit() with xor_bit()

This allows to retire ClearMaskBB[] and use just
one SquareBB[] array to set and clear a bit.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/move.cpp

@@ -98,7 +98,7 @@ const string move_to_san(Position& pos, Move m) {
           // Disambiguation if we have more then one piece with destination 'to'
           // note that for pawns is not needed because starting file is explicit.
           attackers = pos.attackers_to(to) & pos.pieces(pt, pos.side_to_move());
-          clear_bit(&attackers, from);
+          xor_bit(&attackers, from);
           ambiguousMove = ambiguousFile = ambiguousRank = false;
 
           while (attackers)