Remove unnecessary private extern crate

In #2278 I mistakenly left in the `use extern crate` after removing the
`pub` from it - while not incorrect it is unnecessary.

bitcoin/src/lib.rs

@@ -59,10 +59,6 @@ pub extern crate base64;
 /// Encodes and decodes the Bech32 forrmat.
 pub extern crate bech32;
 
-/// Bitcoin's libbitcoinconsensus with Rust binding.
-#[cfg(feature = "bitcoinconsensus")]
-extern crate bitcoinconsensus;
-
 /// Rust implementation of cryptographic hash function algorithems.
 pub extern crate hashes;