Fix radio input test in Chrome

It seems like the `form="pluto"` was throwing it off and making the input live outside the form it should have been contained in, causing it to uncheck A. I added it intending to test the form attribute but ended up not needing it so removing it should be fine. (The tests were passing in phantomjs since it doesn't support the `form` attribute and simply ignored it.)

Test Plan:
grunt test; grunt test --debug

src/dom/components/__tests__/ReactDOMInput-test.js

@@ -61,8 +61,7 @@ describe('ReactDOMInput', function() {
             <input ref="b" type="radio" name="fruit" />B
 
             <form>
-              <input ref="c" type="radio" name="fruit" form="pluto"
-                defaultChecked={true} />
+              <input ref="c" type="radio" name="fruit" defaultChecked={true} />
             </form>
           </div>
         );