Remove unofficial signature of `children`

jQuery's implementation of the `children` method accepts an optional
selector--it does not treat Number values differently.

From the jQuery API documentation of `children`:

> # .children( [selector ] )
>
> Description: Get the children of each element in the set of matched
> elements, optionally filtered by a selector.

http://api.jquery.com/children/
Merge pull request #584 from jugglinmike/fix-children

Remove unofficial signature of `children`