DEV: Delete unused site settings (#23353)

In a previous commit these site settings were removed from the codebase
because they were identified as unused settings. This commit removes
these settings from the db in case they existed in the site settings
table.

Follow up to: da389d7844f8895ea3531c61996a988e4d7b70e5

db/migrate/20230831153649_delete_unused_site_settings.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class DeleteUnusedSiteSettings < ActiveRecord::Migration[7.0]
+  def up
+    execute <<~SQL
+      DELETE
+      FROM
+        "site_settings"
+      WHERE
+        "name" IN (
+          'rate_limit_new_user_create_topic',
+          'enable_system_avatars',
+          'check_for_new_features',
+          'allow_user_api_keys'
+        )
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end