Auto merge of rust-lang#127026 - Urgau:cleanup-bootstrap-check-cfg, r=Kobzol

Cleanup bootstrap check-cfg

This PR cleanup many custom `check-cfg` in bootstrap that have been accumulated over the years.

As well as updating some outdated comments.

Author: bors

alloc/Cargo.toml

@@ -47,8 +47,6 @@ optimize_for_size = ["core/optimize_for_size"]
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
-# for rust-lang/rust. But for users of `-Zbuild-std` it does.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(no_global_oom_handling)',

core/Cargo.toml

@@ -39,8 +39,6 @@ debug_refcell = []
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
-# for rust-lang/rust. But for users of `-Zbuild-std` it does.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(no_fp_fmt_parse)',

std/Cargo.toml

@@ -100,8 +100,6 @@ test = true
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-# x.py uses beta cargo, so `check-cfg` entries do not yet take effect
-# for rust-lang/rust. But for users of `-Zbuild-std` it does.
 check-cfg = [
     'cfg(bootstrap)',
     'cfg(target_arch, values("xtensa"))',