Position: fix a couple of Intel compiler warnings

Plus usual trailing whitespace.

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

src/movepick.cpp

@@ -172,8 +172,6 @@ Move MovePicker::get_next_move() {
         return MOVE_NONE;
     }
   }
-  assert(false);
-  return MOVE_NONE;
 }
 
 

src/position.cpp

@@ -410,7 +410,6 @@ bool Position::piece_attacks_square(Square f, Square t) const {
   case WR: case BR: return piece_attacks_square<ROOK>(f, t);
   case WQ: case BQ: return piece_attacks_square<QUEEN>(f, t);
   case WK: case BK: return piece_attacks_square<KING>(f, t);
-  default:          return false;
   }
   return false;
 }
@@ -1337,7 +1336,6 @@ void Position::undo_castle_move(Move m) {
   // Position::undo_move.  In particular, the side to move has been switched,
   // so the code below is correct.
   Color us = side_to_move();
-  Color them = opposite_color(us);
 
   // Find source squares for king and rook
   Square kfrom = move_from(m);