Remove unused libc feature gate

Libc isn't used by alloc.
And std and panic_* use libc from crates.io now,
which isn't feature gated.

library/alloc/src/lib.rs

@@ -103,7 +103,6 @@
 #![feature(inplace_iteration)]
 #![feature(lang_items)]
 #![feature(layout_for_ptr)]
-#![feature(libc)]
 #![feature(map_first_last)]
 #![feature(map_into_keys_values)]
 #![feature(maybe_uninit_ref)]

library/panic_abort/src/lib.rs

@@ -12,7 +12,6 @@
 #![panic_runtime]
 #![allow(unused_features)]
 #![feature(core_intrinsics)]
-#![feature(libc)]
 #![feature(nll)]
 #![feature(panic_runtime)]
 #![feature(staged_api)]

library/panic_unwind/src/lib.rs

@@ -19,7 +19,6 @@
 )]
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
-#![feature(libc)]
 #![feature(nll)]
 #![feature(panic_unwind)]
 #![feature(staged_api)]

library/std/src/lib.rs

@@ -268,7 +268,6 @@
 #![feature(integer_atomics)]
 #![feature(into_future)]
 #![feature(lang_items)]
-#![feature(libc)]
 #![feature(link_args)]
 #![feature(linkage)]
 #![feature(llvm_asm)]