Explicitly mention our lack of VAT registration on invoices

Even though that's implied by not listing a VAT number, we keep getting
a lot of questions about this, so state it clearly. Suggested by
ads, wording by Dave.
Remove the old invoice system from the admin interface

Since it's readonly now, there is no value to have it in the admin system,
and I keep clicking into the old one instead of hte new one myself by mistake,
so get rid of the links and contents.