remove arrowIcon default value

Author: HeskeyBaozi

README.md

@@ -112,7 +112,7 @@ ReactDOM.render(collapse, container);
       <tr>
           <td>arrowIcon</td>
           <td>React.Node</td>
-          <th>''</th>
+          <th></th>
           <td>specific the custom arrow icon.</td>
       </tr>
     </tbody>

src/Collapse.jsx

@@ -142,7 +142,6 @@ Collapse.defaultProps = {
   onChange() {},
   accordion: false,
   destroyInactivePanel: false,
-  arrowIcon: '',
 };
 
 Collapse.Panel = CollapsePanel;