DEV: Put back the model step in HandleCategoryUpdated service

Now that `ActiveRecord` relations are properly handled in a `model`
step, putting this step back will allow the service to stops its
execution if there are no users to be removed without calling the pretty
big SQL query contained in the `CalculateMembershipsForRemoval` action.

plugins/chat/app/services/chat/auto_remove/handle_category_updated.rb

@@ -19,6 +19,7 @@ module Chat
       policy :chat_enabled
       model :category
       model :category_channel_ids
+      model :users
       step :remove_users_without_channel_permission
       step :publish
 
@@ -46,18 +47,21 @@ module Chat
         Chat::Channel.where(chatable: category).pluck(:id)
       end
 
-      def remove_users_without_channel_permission(category_channel_ids:)
+      def fetch_users(category_channel_ids:)
+        User
+          .real
+          .activated
+          .not_suspended
+          .not_staged
+          .joins(:user_chat_channel_memberships)
+          .where("user_chat_channel_memberships.chat_channel_id IN (?)", category_channel_ids)
+          .where("NOT admin AND NOT moderator")
+      end
+
+      def remove_users_without_channel_permission(users:, category_channel_ids:)
         memberships_to_remove =
           Chat::Action::CalculateMembershipsForRemoval.call(
-            scoped_users_query:
-              User
-                .real
-                .activated
-                .not_suspended
-                .not_staged
-                .joins(:user_chat_channel_memberships)
-                .where("user_chat_channel_memberships.chat_channel_id IN (?)", category_channel_ids)
-                .where("NOT admin AND NOT moderator"),
+            scoped_users_query: users,
             channel_ids: category_channel_ids,
           )