Fix Lint/UselessAssignment cop (#27472)

app/services/activitypub/process_status_update_service.rb

@@ -97,8 +97,6 @@ class ActivityPub::ProcessStatusUpdateService < BaseService
       end
     end
 
-    added_media_attachments = @next_media_attachments - previous_media_attachments
-
     @status.ordered_media_attachment_ids = @next_media_attachments.map(&:id)
 
     @media_attachments_changed = true if @status.ordered_media_attachment_ids != previous_media_attachments_ids