This config was added in 207de019dc ("libary: Forward
compiler-builtins "asm" and "mangled-names" feature") but it does not
appear this has ever been used. The PR adding it (RUST-78472) says that
this was exposed to help with configuration and points at the [Hermit
Cargo config], but as far as I can tell, this feature name has never
been mentioned in that repository's git history.
Thus, clean up a seemingly unneeded feature.
[Hermit Cargo config]: ab2b830930/.cargo/config
51c5f191de