DEV: remove experimental_sidebar_messages_count_enabled_groups (#34167)

Removes the temporary `experimental_sidebar_messages_count_enabled_groups` site setting.

Reverts #33774.

db/migrate/20250808064354_remove_experimental_sidebar_messages_count_enabled_groups_site_setting.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+class RemoveExperimentalSidebarMessagesCountEnabledGroupsSiteSetting < ActiveRecord::Migration[8.0]
+  def up
+    execute "DELETE FROM site_settings WHERE name = 'experimental_sidebar_messages_count_enabled_groups'"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end