Removing a redundant/incorrect cast to JSDynamicObject.

(cherry picked from commit 8addafc)

Author: iamstolis

graal-js/src/com.oracle.truffle.js/src/com/oracle/truffle/js/builtins/ArrayPrototypeBuiltins.java

@@ -2444,7 +2444,7 @@ private static final class InnerFlattenCallNode extends JavaScriptRootNode {
             @Override
             public Object execute(VirtualFrame frame) {
                 Object[] arguments = frame.getArguments();
-                JSDynamicObject resultArray = (JSDynamicObject) arguments[0];
+                Object resultArray = arguments[0];
                 Object element = arguments[1];
                 long elementLen = (long) arguments[2];
                 long targetIndex = (long) arguments[3];