Rollup merge of #153622 - RalfJung:rm-soft-unstable, r=JonathanBrouwer

remove concept of soft-unstable features

Implements rust-lang/compiler-team#972

@rust-lang/libs-api last chance to speak up before this feature is gone :)

Fixes rust-lang/rust#153572
Fixes rust-lang/rust#64266

Author: JonathanBrouwer