Describe how to remove components when update fails

[shepmaster]

Describe the problem you are trying to solve

When upgrading to a Rust version missing currently installed components, we get the error message including the text

If you require these components

This has great text for what to do in this case, but we don't discuss the other case!

(Filed due to a Stack Overflow question: Unable to update stable rustup toolchain: some components unavailable for target i386-apple-ios, armv7s-apple-ios)

Describe the solution you'd like

It would be good to describe how to remove the components if we don't need them anymore (ideally copy-pastable).

Something like:

rustup component remove --toolchain $TC --target $TG $NAME

For each conflicting component.

[kinnison]

This looks moderately easy to do, especially since there's a model for doing it. I'd be pleased to have someone come along to our #wg-rustup channel on the rust-lang discord and I'll mentor the work.