grsecurity module: remove reference to systemd-sysctl

First, that's not what the service is called, and secondly it's
most likely irrelevant to the user.

nixos/modules/security/grsecurity.nix

@@ -131,9 +131,6 @@ in
             <literal>kernel.grsecurity.grsec_lock</literal> to
             non-zero as soon as all sysctl options are set. *THIS IS
             EXTREMELY IMPORTANT*!
-
-            If disabled, this also turns off the
-            <literal>systemd-sysctl</literal> service.
           '';
         };