Optimization: Remove classes that aren't in the stylesheet.

If an element has a classname that isn't in the source stylesheet, that class should be removed by the rewriter.

We can save the classes that are used in only the stylesheets during the `initKnownIdents` initializer and then create `MarkAttributeValueObsolete` actions for the classnames that are never found.

Start at the initKnownIdents initializer and trace how classes used by stylesheets are recorded. Then inspect how the rewriter creates MarkAttributeValueObsolete actions for class names; done means classes absent from the source stylesheets are removed while stylesheet-used classes remain.

Written by the indexing model from the issue text.