elimination of filter_form field type

The attached patch eliminates the filter_form field type. To me, there is not enough new functionality here to warrant a separate type. The increased control of appearance (via themeability) is better handled with an HTML id attribute and some CSS. The current implementation also requires the Input Format radio buttons to be themed independently from all other radio buttons which, unless there is some drastic change in HTML (it looks similar to me) or the type is likely to be used often, seems like overkill.