[development] forms api issue: duplicate class attributes in form elements

[Nedjo Rogers]

Reviews are needed on issue http://drupal.org/node/54104.

We're getting duplicate (invalid) class attributes on form elements because 
classes are being added two different ways: as part of an element's 
$attributes array ($element['#attributes']['class']) and through hard-coded 
output in the element theme_ functions.  This error breaks display and any 
Javascript dependent on form element classes.