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

[kenjis]

Description
Revert #3423
Fixes #5818

Why

• The autoload.psr-4 causes problems:
  • Cannot change app folder name. Because the composer's path overwrite the config in Config\Autoload. See https://forum.codeigniter.com/thread-81153.html
  • Causes error when defining new namespace under app/. See Bug: defining new namespace under app/ causes Cannot declare class ..., because the name is already in use #5818
  • These are very difficult to debug.
• Only app starter has the configs. So it is not consistent.

If there is a user who wants to use Composer autoloader optimization (dump-autoload -o) for App namespace class files, they could add it in autoload.psr-4 by themself.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• [] Conforms to style guide

[kenjis]

@samsonasik Thank you!