Remove extern crate core statement

Now that we have an MSRV of 1.41.1 we no longer need `extern crate
core`, remove it.

src/lib.rs

@@ -62,9 +62,6 @@ extern crate alloc;
 #[cfg(feature = "no-std")]
 extern crate core2;
 
-#[cfg(any(feature = "std", test))]
-extern crate core; // for Rust 1.29 and no-std tests
-
 // Re-exported dependencies.
 #[macro_use] pub extern crate bitcoin_hashes as hashes;
 pub extern crate secp256k1;