Content Type showing up in edit form?

I have a library with a couple of custom content types all inheriting from Document Set and in the edit form the Content Type field shows up. I would rather remove it because it doesn't make any sense for these items to change content type after they are created. I added<FieldRef ID="{c042a256-787d-4a6f-8a8a-cf6ab767f12d}" Name="ContentType" ShowInEditForm="FALSE"/> to the content type and list definition, but it's still showing up. How do I remove the content type field from the edit form?