Also delete extern crate unwind

src/beneath-std.md

@@ -22,10 +22,6 @@ in the same format as C (aside from the exact integer types being used):
 #![allow(internal_features)]
 #![no_std]
 
-// Necessary for `panic = "unwind"` builds on some platforms.
-#![feature(panic_unwind)]
-extern crate unwind;
-
 use core::panic::PanicInfo;
 
 // Entry point for this program.
@@ -53,10 +49,6 @@ compiler's name mangling too:
 #![no_std]
 #![no_main]
 
-// Necessary for `panic = "unwind"` builds on some platforms.
-#![feature(panic_unwind)]
-extern crate unwind;
-
 use core::ffi::{c_char, c_int};
 use core::panic::PanicInfo;