nixos/nextcloud: remove invalid --database-table-prefix option

This doesn't work anymore and thus breaks the installation leaving a
broken `/var/lib/nextcloud`.

It isn't a big deal since we set this value in the override config
before, so the correct table-prefix is still used. In order to confirm
that, I decided to add a custom prefix to the basic test.

(cherry picked from commit eaeb4fe04ee5a5ffae4f7dc03d3f072d59d80337)

nixos/tests/nextcloud/basic.nix

@@ -37,6 +37,7 @@ in {
         config = {
           # Don't inherit adminuser since "root" is supposed to be the default
           inherit adminpass;
+          dbtableprefix = "nixos_";
         };
         autoUpdateApps = {
           enable = true;