Remove unstable `libc` feature on rustc-dep-of-std
Remove unstable `libc` feature on rustc-dep-of-std

No longer needed and this is done via other means in upstream rustc

rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc