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

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

part of #13629

Closes #13246

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[weihanglo]

Could you do documentation update as well?

• src/doc/src/reference/cargo-targets.md could just indicate that “This option is deprecated and unused”.
• Change this line to “mostly for support of proc-macros”.
• Any other comments or function names containing "plugin" that needs to be removed. You can hold of this a bit, as it might require extra efforts.

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Personally the change is fine, though I still think it is great to ship this along with the version introduction edition 2024 (not tie to edition just merge this in the same version).

[weihanglo]

Thanks! Could you squash doc commits into one, and test commits into another?

[weihanglo]

Thanks!

It might not be necessary, though I'd like to wait a little longer. I'll merge this once nightly 1.81 is out.