guix: self: Do not install (gnu system install).

As we do not want to add a dependency to newt and the graphical installer
in (guix self), do not install (gnu system install).

* guix/self.scm (*system-modules*): Remove (gnu system install) from
"guix-system" scheme-node.

guix/self.scm

@@ -604,7 +604,11 @@ Info manual."
     (scheme-node "guix-system"
                  `((gnu system)
                    (gnu services)
-                   ,@(scheme-modules* source "gnu/system")
+                   ,@(filter-map
+                      (match-lambda
+                        (('gnu 'system 'install) #f)
+                        (name name))
+                      (scheme-modules* source "gnu/system"))
                    ,@(scheme-modules* source "gnu/services"))
                  (list *core-package-modules* *package-modules*
                        *extra-modules* *core-modules*)
@@ -806,7 +810,6 @@ Info manual."
                ;; made relative to a nonexistent anonymous module.
                #:splice? #t))
 
-
 
 ;;;
 ;;; Building.