refactor: remove App namespace in AutoloadConfig

It is set in Config\Autoload, and it may be renamed.

Author: kenjis

system/Config/AutoloadConfig.php

@@ -92,7 +92,6 @@ class AutoloadConfig
      */
     protected $corePsr4 = [
         'CodeIgniter' => SYSTEMPATH,
-        'App'         => APPPATH, // To ensure filters, etc still found,
         'Config'      => APPPATH . 'Config',
     ];