Fix cloneFragment conditional handling of clipboardData.setData (#2298)

Seems the issue was introduced here: dc95ad66a5 (diff-dfb9fbad6106fa548b8335a644d03e49L109)

Remvoing the `return` ended  up calling `callback` twice instead of the intended once and both branches being followed

packages/slate-react/src/utils/clone-fragment.js

@@ -114,6 +114,7 @@ function cloneFragment(event, editor, callback = () => undefined) {
     event.clipboardData.setData(FRAGMENT, encoded)
     event.clipboardData.setData(HTML, div.innerHTML)
     callback()
+    return
   }
 
   // COMPAT: For browser that don't support the Clipboard API's setData method,