If only a single category remains, there shouldn't be a fieldset

The standard taxonomy behaviour is to include a 'Categories' fieldset when the number of categories exceeds one. I.e., if there is only a single category that applies to this content type, then the fieldset is not included, but the term selection widget is simply included alongside all the other fields in the node form.

If OG Vocab strips all but a single category from a node form, then we should remove the fieldset properties from $form['taxonomy'] to be consistent with this behaviour. Patch attached.