backups: backup NixOS uid, gid mappings

Now that service uid, gid mappings are included in the backups, along
with the service data dirs, we can remove 'chown -R' for
clightning and liquidd data dirs.

Note that we used 'chown -R' only for these two services, while this
approach would have been relevant for all services with data dirs.

modules/liquid.nix

@@ -221,7 +221,6 @@ in {
       after = [ "bitcoind.service" ];
       wantedBy = [ "multi-user.target" ];
       preStart = ''
-        chown -R '${cfg.user}:${cfg.group}' '${cfg.dataDir}'
         install -m 640 ${configFile} '${cfg.dataDir}/elements.conf'
         {
           echo "rpcpassword=$(cat ${secretsDir}/liquid-rpcpassword)"