docs: Remove extern crate

Now we have edition 2018 we no longer need to use `extern crate` in the
docs examples.

Author: tcharding

src/lib.rs

@@ -53,9 +53,6 @@
 //! ## Deriving an address from a descriptor
 //!
 //! ```rust
-//! extern crate bitcoin;
-//! extern crate miniscript;
-//!
 //! use std::str::FromStr;
 //! use miniscript::{DescriptorTrait};
 //!

src/miniscript/mod.rs

@@ -172,9 +172,6 @@ impl<Ctx: ScriptContext> Miniscript<Ctx::Key, Ctx> {
     /// ## Decode/Parse a miniscript from script hex
     ///
     /// ```rust
-    /// extern crate bitcoin;
-    /// extern crate miniscript;
-    ///
     /// use miniscript::Miniscript;
     /// use miniscript::{Segwitv0, Tap};
     /// use miniscript::bitcoin::secp256k1::XOnlyPublicKey;