Remove unstable cfg target(...) compact feature

[Urgau]

This PR removes the unstable cfg_target_compact feature from #96901, which permits compact target(...) cfgs (e.g. cfg(target(os = "linux")) -> cfg(target_os = "linux")).

The feature:

• is only used in one crate: GitHub Search (not even in the compiler)
• has received no activity on the tracking issue
• has not received any stabilization request since it's nightly introduction (~2yrs ago)
• was part of a bigger feature that was already withdrawn
• is limited to target_* cfgs
• is not implemented by Cargo

All this to say that I don't think the feature pulls its weight.

@rustbot label +I-lang-nominated
Closes #96901

[rustbot]

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[lqd]

Wasn’t this helping to remove some use cases for build scripts?

[Urgau]

The part that was supposed to help with build script was the cfg(target = "...") part of the RFC, but that part was already withdrawn.

[lqd]

IIRC the target abi was only matchable in build scripts, is that still the case today (with and without this PR)? To be able to lower the need for build scripts in the ecosystem we need cfgs to be at least isomorphic…

[Urgau]

This PR doesn't change anything for matching on specific target cfgs components. This PR only removes the shorthand cfg(target(os = "linux")) -> cfg(target_os = "linux").

As for the target abi, the target_abi cfg was stabilized in 1.78.

[lqd]

Good. Thank you for the clarifications.