feat(group): hide add membership feature for group admin

Author: rahmatrhd

app/views/groups/show.html.slim

@@ -68,7 +68,7 @@
       br
       = "*This group does not have any group members" if @group_users.length == 0
       br
-      - if current_user.admin or @group.admin?(current_user)
+      - if current_user.admin
         .h7 Assign members
         = form_tag  add_user_to_group_path(@group.id), method: :post do
           .row