Don't allow users to delete their default addressbook

[elimae17]

A user of my Nextcloud instance deleted their default addressbook and was then unable to create new contacts. I fixed the problem by recreating it with the command occ dav:create-addressbook <user> contacts but I don't think it should be possible to delete the default address book in the first place, just as it is not possible to delete the system address book.

[GVodyanov]

Hey! Thanks for the idea! If you feel like giving a shot at creating a PR we would be happy to help.