Hello,
Certain fields in recordSets are considered crucial, the system relies on them and removing/renaming them is a bad idea. First and Last names being one of the most crucial, I believe.
There is a way however, to both make the system think the field is there and yet not bother the user with that field. You'll have to do this directly from the database:
Find table contact_field
On row with field "Last Name" set type to "hidden". This should get you the desired effect.
Alternatively, you can also use translations to change the field label, without touching the underlying mechanism. This however will have the consequence of carrying out all mechanics of the field, so if you rename "Last Name" to, say "Eye Colour", this field will still be used to create labels for contacts (Joe Doe will become Joe Blue).
Kind regards,
Arek