fix(toml): remove `lib.plugin` key support and make it warning

[heisen-li]

What does this PR try to resolve?

Remove lib.plugin key, making it an "unused key" warning.

Remove some of the tests, which should look useless (I hope I'm understanding this

• [x] Remove key, and related tests.
• [x] Adjust the documentation about the plugin.
• [ ] Some of the comments and function names have not yet finished being modified.

part of #13629

Closes #13246