PERF: Reduce mem allocation of Chat::AutoRemove::HandleCategoryUpdated (#28393)

This is a follow-up to 671f40ce07ebda14e4655cb85ce4794b859023f0 and
ed11ee9d057160e5c1b0d1a86c9e94582d8fafd0.

While the optimisations in the previous commits were sound, it did not
resolve the memory bloat we were seeing. It turns out that we call
`.blank?` on the model's result if the model has not been marked
optional. The problem with this is that if the model returns an
ActiveRecord relation, calling `.blank?` on the relation basically loads
everything into memory.

Therefore, this commit removes `users` as a model in the  since it really isn't
a model but just a relation.

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

@@ -19,7 +19,6 @@ module Chat
       policy :chat_enabled
       model :category
       model :category_channel_ids
-      model :users
       step :remove_users_without_channel_permission
       step :publish
 
@@ -47,21 +46,18 @@ module Chat
         Chat::Channel.where(chatable: category).pluck(:id)
       end
 
-      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:)
+      def remove_users_without_channel_permission(category_channel_ids:)
         memberships_to_remove =
           Chat::Action::CalculateMembershipsForRemoval.call(
-            scoped_users_query: users,
+            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"),
             channel_ids: category_channel_ids,
           )