Remove libc from beneath-std, it's not actually required anymore

src/beneath-std.md

@@ -4,23 +4,6 @@ This section documents features that are normally provided by the `std` crate an
 that `#![no_std]` developers have to deal with (i.e. provide) to build
 `#![no_std]` binary crates.
 
-## Using `libc`
-
-In order to build a `#[no_std]` executable we will need `libc` as a dependency.
-We can specify this using our `Cargo.toml` file:
-
-```toml
-[dependencies]
-libc = { version = "0.2.146", default-features = false }
-```
-
-Note that the default features have been disabled. This is a critical step -
-**the default features of `libc` include the `std` crate and so must be
-disabled.**
-
-Alternatively, we can use the unstable `rustc_private` private feature together
-with an `extern crate libc;` declaration as shown in the examples below.
-
 ## Writing an executable without `std`
 
 We will probably need a nightly version of the compiler to produce
@@ -35,17 +18,14 @@ The function marked `#[start]` is passed the command line parameters
 in the same format as C (aside from the exact integer types being used):
 
 ```rust
-#![feature(start, lang_items, core_intrinsics, rustc_private)]
+#![feature(start, lang_items, core_intrinsics)]
 #![allow(internal_features)]
 #![no_std]
 
 // Necessary for `panic = "unwind"` builds on some platforms.
 #![feature(panic_unwind)]
 extern crate unwind;
 
-// Pull in the system libc library for what crt0.o likely requires.
-extern crate libc;
-
 use core::panic::PanicInfo;
 
 // Entry point for this program.
@@ -68,7 +48,7 @@ correct ABI and the correct name, which requires overriding the
 compiler's name mangling too:
 
 ```rust
-#![feature(lang_items, core_intrinsics, rustc_private)]
+#![feature(lang_items, core_intrinsics)]
 #![allow(internal_features)]
 #![no_std]
 #![no_main]
@@ -77,9 +57,6 @@ compiler's name mangling too:
 #![feature(panic_unwind)]
 extern crate unwind;
 
-// Pull in the system libc library for what crt0.o likely requires.
-extern crate libc;
-
 use core::ffi::{c_char, c_int};
 use core::panic::PanicInfo;