chore: remove App\ and Config\ in autoload.psr-4 in app starter composer.json

Revert #3423

Problems:
- Cannot change `app` folder name. Because the composer's path overwrite the config in Config\Autoload.
- Causes error when defining new namespace under app/. See #5818

Author: kenjis

admin/starter/composer.json

@@ -17,10 +17,6 @@
         "ext-fileinfo": "Improves mime type detection for files"
     },
     "autoload": {
-        "psr-4": {
-            "App\\": "app",
-            "Config\\": "app/Config"
-        },
         "exclude-from-classmap": [
             "**/Database/Migrations/**"
         ]