Remove unnecessary 'NA'-fallback for getTextContentAccessor()

The `'NA'`-fallback is not used in https://github.com/facebook/react/blob/master/src/eventPlugins/CompositionEventPlugin.js#L175

I don't see why we need it at all? If the browser doesn't support a text content accessor then there's nothing we can do, assigning to `'NA'` or `undefined` should be a meaningless distinction with the same result.