Rollup merge of #141925 - cuviper:vestigial-bootstrap, r=workingjubilee

Remove bootstrap cfgs from library/

These `cfg(bootstrap)` are always false now that rust-lang/rust#119899 has landed, and likewise `cfg(not(bootstrap))` is always true. Therefore, we don't need to wait for the usual stage0 bump to clean these up.

Author: matthiaskrgr