FIX: Skip validation on enforcing second factor change if the value is "no"

Without this, this site setting breaks plugin tests if local logins are
disabled in the plugin.

lib/site_settings/validations.rb

@@ -151,6 +151,7 @@ module SiteSettings::Validations
 
   def validate_enforce_second_factor(new_val)
     return if SiteSetting.enable_local_logins
+    return if new_val == "no"
     validate_error :second_factor_cannot_be_enforced_with_disabled_local_login
   end