remove extern crate pkg_config from examples

No longer needed as of Rust 2018

Author: Be-ing

src/lib.rs

@@ -38,8 +38,6 @@
 //! Find the system library named `foo`, with minimum version 1.2.3:
 //!
 //! ```no_run
-//! extern crate pkg_config;
-//!
 //! fn main() {
 //!     pkg_config::Config::new().atleast_version("1.2.3").probe("foo").unwrap();
 //! }
@@ -49,8 +47,6 @@
 //! recommended):
 //!
 //! ```no_run
-//! extern crate pkg_config;
-//!
 //! fn main() {
 //!     pkg_config::probe_library("foo").unwrap();
 //! }
@@ -59,8 +55,6 @@
 //! Configure how library `foo` is linked to.
 //!
 //! ```no_run
-//! extern crate pkg_config;
-//!
 //! fn main() {
 //!     pkg_config::Config::new().atleast_version("1.2.3").statik(true).probe("foo").unwrap();
 //! }