Remove unneeded extern crate core

We have edition 2018 now, we no longer need to explicitly use `extern
crate core`.

Author: tcharding

src/lib.rs

@@ -60,9 +60,6 @@ assert_eq!(variant, Variant::Bech32);
 #[cfg(all(not(feature = "std"), not(test)))]
 extern crate alloc;
 
-#[cfg(any(test, feature = "std"))]
-extern crate core;
-
 #[cfg(all(not(feature = "std"), not(test)))]
 use alloc::{string::String, vec::Vec};