Removed setting override system due to confusing behaviour

- Was only used to disable registration when LDAP was enabled.
- Caused saved option not to show on settings page causing confusion.
- Extended setting logic where used to take ldap into account instead of
global override.
- Added warning on setting page to show registration enable setting is
not used while ldap is active.

For #1541

app/Http/Controllers/Auth/RegisterController.php

@@ -89,7 +89,7 @@ class RegisterController extends Controller
      */
     protected function checkRegistrationAllowed()
     {
-        if (!setting('registration-enabled')) {
+        if (!setting('registration-enabled') || config('auth.method') === 'ldap') {
             throw new UserRegistrationException(trans('auth.registrations_disabled'), '/login');
         }
     }