Don't set parent key in get_comment response to the Post parent

In `WP_JSON_Posts->prepare_comment()` if the $context parameter = single we set $fields['parent'] to the parent post (line 1564)....and then we set $fields['parent'] a few lines down to the parent comment (line 1576)? https://github.com/WP-API/WP-API/blob/master/lib/class-wp-json-posts.php#L1564

I don't think we should get the post_parent here at all.  The .com API only gives the comment parent: http://developer.wordpress.com/docs/api/1/get/sites/%24site/comments/%24comment_ID/

@rmccue any reason for this bizarre logic?