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.

nixos/modules/services/web-apps/nextcloud.nix

@@ -502,8 +502,6 @@ in {
               ${if c.dbport != null then "--database-port" else null} = ''"${toString c.dbport}"'';
               ${if c.dbuser != null then "--database-user" else null} = ''"${c.dbuser}"'';
               "--database-pass" = dbpass;
-              ${if c.dbtableprefix != null
-                then "--database-table-prefix" else null} = ''"${toString c.dbtableprefix}"'';
               "--admin-user" = ''"${c.adminuser}"'';
               "--admin-pass" = adminpass;
               "--data-dir" = ''"${cfg.home}/data"'';

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;