refactor: remove Composer autoloading of email helper

[kenjis]

Description
See https://forum.codeigniter.com/showthread.php?tid=88545

The Email helper is used in a limited number of places, and it would be inefficient to load it on every page.

Also, if a library autoloads helpers by Composer, there is no way to stop that loading.
So I have changed my mind, it is better not using helper autoloading by Composer in libraries.
If autoloading is necessary, developers can set it up in Config\Autoload.

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

[datamweb]

two questions,
If we are going to be bound to CI 4.3, isn't it better to use on config/autoload.php for auth_helper?

Regarding filters, we load a lot of filters by default, what do you think?

[kenjis]

If we are going to be bound to CI 4.3, isn't it better to use on config/autoload.php for auth_helper?

Yes, I will send another PR.

[kenjis]

Regarding filters, we load a lot of filters by default, what do you think?

Probably no. Only the applied filters for the request URI are loaded.

[kenjis]

I sent #893