Fixes to improve plone.app.textfield in Lists

In Dexterity, if you have a schema.List of RichText fields (or Object field containing RichText), choosing MIME types doesn't work particularly well.

The first commit stops the format box affecting other fields in the list, the second allows plone.app.textfield (pull request forthcoming) to override the MIME type. ``getContentType()`` cannot handle complicated forms, but given we already know the MIME type it seems easier just to not use it.