Revert "nixos/postfix: fix in secure systemd-nspawn containers"

This reverts commit f2eda9a6a9.
Adding `-p` results in a world-readable file which we don't want for
e.g. `smtp_sasl_password_maps`:

> Keep the SASL client password file in /etc/postfix, and make the file
> read+write only for root to protect the username/password combinations
> against other users.[^1]

[^1]: https://www.postfix.org/SASL_README.html#client_sasl_enable

nixos/modules/services/mail/postfix.nix

@@ -972,7 +972,7 @@ in
             ${lib.concatStringsSep "\n" (
               lib.mapAttrsToList (to: from: ''
                 ln -sf ${from} /var/lib/postfix/conf/${to}
-                ${lib.getExe' cfg.package "postmap"} -o -p /var/lib/postfix/conf/${to}
+                ${lib.getExe' cfg.package "postmap"} /var/lib/postfix/conf/${to}
               '') cfg.mapFiles
             )}