The class wikigeneratedid on headings is not removed when there is no id

When pressing "enter" on a heading in CKEditor, the heading is duplicated but while both of them have the class wikigeneratedid, only one of them has an id. The XHTML parser only removes the class when there is an id. Therefore, the class value ends up in the XWiki syntax source code. This is not a huge problem, the next time the WYSIWYG editor is used, the class value will be removed again (as then an id exists), but it's still not nice. The code is unchanged since XRENDERING-476 so the bug is probably as old as XWiki 9.7 RC1.

To fix this, the automatically generated class values should always be removed, regardless of the presence of the id attribute.