services: root-file-system: Remove reference to 'stop-logging'.

* gnu/services/base.scm (%root-file-system-shepherd-service): In 'stop'
method, remove reference to 'stop-logging'.  That procedure is gone in
Shepherd 0.10.0, leading to an unbound-variable exception.
Additionally, calling is unnecessary since 0.4.0, where shepherd logs to
syslog (thus the open file descriptor is not backed by an on-disk file).

gnu/services/base.scm

@@ -324,11 +324,7 @@ system objects.")))
              (sync)
 
              (let ((null (%make-void-port "w")))
-               ;; Close 'shepherd.log'.
-               (display "closing log\n")
-               ((@ (shepherd comm) stop-logging))
-
-               ;; Redirect the default output ports..
+               ;; Redirect the default output ports.
                (set-current-output-port null)
                (set-current-error-port null)