remove "instanceof Pawn"

app/src/main/java/chess/model/visitor/KingIdentifier.java

@@ -1,6 +1,7 @@
 package chess.model.visitor;
 
 import chess.model.Color;
+import chess.model.Piece;
 import chess.model.PieceVisitor;
 import chess.model.pieces.*;
 
@@ -12,6 +13,12 @@ public class KingIdentifier implements PieceVisitor<Boolean> {
 		this.color = color;
 	}
 
+	public boolean isKing(Piece piece) {
+		if (piece == null)
+			return false;
+		return visit(piece);
+	}
+
 	@Override
 	public Boolean visitPiece(Bishop bishop) {
 		return false;