Stop 'Notice: Use of undefined constant jQuery' to be raised.

I got the notice with `define('WP_DEBUG', true);` in `wp-config.php`.

`wp_register_script()` skips adding if the script specified with handler
exists already.
For this reason, `if` construct before `wp_register_script()` may not be
needed.

* Code on WordPress 3.0
http://core.trac.wordpress.org/browser/tags/3.0/wp-includes/class.wp-dependencies.php#L135
* Code on WordPress 3.5.1
http://core.trac.wordpress.org/browser/tags/3.5.1/wp-includes/class.wp-dependencies.php#L128