Remove --all-targets flag from x check

[Kobzol]

Chris was trying to get x check library --all-targets working in https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Checking.20std.20tests.20without.20building/with/613735399.

I took a look into this and realized that the x check --all-targets flag is literally unused - nothing in bootstrap was reading it. So the compiler and library were never checked with --all-targets, but on the other hands all tools are checked with --all-targets (or equivalent) by default, even without the flag.

So I think that we have two options:

1. Actually respect x check --all-targets, and only check all targets for tools when the flag is passed. So checking tools would do less work by default.
2. Remove the flag, and always check all targets. So checking the compiler will do a bit more work. This is what I decided to do in this PR.

Note that checking std with --all-targets is broken, it just doesn't seem to work when we just pass --all-targets to it.

Happy to hear thoughts on this.

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

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[Zalathar]

Does this end up checking the compiler crates twice, once as a library crate and once again with cfg(test) for unit tests?

(That might still be desirable, but if so then it’s effectively double the work.)