Merge rust-bitcoin/rust-bitcoin#1094: Remove extern crate core statement

35edcaa Remove extern crate core statement (Tobin C. Harding)

Pull request description:

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

ACKs for top commit:
  apoelstra:
    ACK 35edcaa
  Kixunil:
    ACK 35edcaa

Tree-SHA512: 0efa0f05d3e9f797c493757fce6ca7703dd8439dbce4e9ff8113494fc71691ab4171c6f0fb5239b57ff5f0e223d48b057265a3df79898a399ca455040580aab2

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;