fix: remove Config\Validation::$_username

Author: kenjis

src/Controllers/LoginController.php

@@ -64,7 +64,7 @@ public function loginAction(): RedirectResponse
     protected function getValidationRules(): array
     {
         return setting('Validation.login') ?? [
-            //'username' => setting('Validation._username') ?? $this->getUsernameRules(),
+            //'username' => static::getUsernameRules(),
             'email'    => static::getEmailRules(),
             'password' => 'required',
         ];

src/Controllers/RegisterController.php

@@ -130,10 +130,8 @@ protected function getUserEntity(): User
      */
     protected function getValidationRules(): array
     {
-        $usernameRules = setting('Validation._username') ?? LoginController::getUsernameRules();
-
         $registrationUsernameRules = array_merge(
-            $usernameRules,
+            LoginController::getUsernameRules(),
             ['is_unique[users.username]']
         );
         $registrationEmailRules = array_merge(