head-node: Remove ‘cuirass-configuration’ field that no longer exists.

This field was removed in Guix commit
7d9ad8f3a695b6d813f484625681263f4509f181 (2nd May 2024).

* head-node.scm (cuirass-configuration)[use-substitutes?]: Remove.

head-node.scm

@@ -497,7 +497,6 @@
                  (service cuirass-service-type
                           (cuirass-configuration
                            (cuirass latest:cuirass)
-                           (use-substitutes? #t)
 
                            ;; By default, poll repositories relatively
                            ;; infrequently.  Send POST requests on