system: Remove useless 'members' field of 'user-group'.

* gnu/system/shadow.scm (<user-group>)[members]: Remove field.
* gnu/system/install.scm (installation-os)[users]: Remove 'members'
  fields.  Use 'supplementary-groups' for 'guest'.
* build-aux/hydra/demo-os.scm (users): Likewise.
* gnu/services/base.scm (guix-service): Remove 'members' field.

build-aux/hydra/demo-os.scm

@@ -51,19 +51,18 @@
 
  (users (list (user-account
                (name "guest")
-               (group "wheel")
+               (group "users")
+               (supplementary-groups '("wheel"))  ; allow use of sudo
                (password "")
                (comment "Guest of GNU")
                (home-directory "/home/guest"))))
  (groups (list (user-group (name "root") (id 0))
                (user-group
                 (name "wheel")
-                (id 1)
-                (members '("guest")))             ; allow 'guest' to use sudo
+                (id 1))
                (user-group
                 (name "users")
-                (id 100)
-                (members '("guest")))))
+                (id 100))))
 
  (issue "
 This is an alpha preview of the GNU system.  Welcome.

gnu/services/base.scm

@@ -391,8 +391,6 @@ hydra.gnu.org are used by default."
              (user-accounts accounts)
              (user-groups (list (user-group
                                  (name builder-group)
-                                 (members (map user-account-name
-                                               user-accounts))
 
                                  ;; Use a fixed GID so that we can create the
                                  ;; store with the right owner.

gnu/system/install.scm

@@ -111,19 +111,18 @@ Use Alt-F2 for documentation.
 
     (users (list (user-account
                   (name "guest")
-                  (group "wheel")
+                  (group "users")
+                  (supplementary-groups '("wheel"))  ; allow use of sudo
                   (password "")
                   (comment "Guest of GNU")
                   (home-directory "/home/guest"))))
     (groups (list (user-group (name "root") (id 0))
                   (user-group
                    (name "wheel")
-                   (id 1)
-                   (members '("guest")))          ; allow 'guest' to use sudo
+                   (id 1))
                   (user-group
                    (name "users")
-                   (id 100)
-                   (members '("guest")))))
+                   (id 100))))
 
     (issue %issue)
 

gnu/system/shadow.scm

@@ -40,7 +40,6 @@
             user-group-name
             user-group-password
             user-group-id
-            user-group-members
 
             default-skeletons
             skeleton-directory))
@@ -70,8 +69,7 @@
   user-group?
   (name           user-group-name)
   (password       user-group-password (default #f))
-  (id             user-group-id (default #f))
-  (members        user-group-members (default '())))
+  (id             user-group-id (default #f)))
 
 (define (default-skeletons)
   "Return the default skeleton files for /etc/skel.  These files are copied by