lose "extern crate" everywhere (Rust 2018 edition)

ffmpeg/build.rs

@@ -28,9 +28,6 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-extern crate cc;
-extern crate pkg_config;
-
 fn main() {
     let libraries = [
         pkg_config::Config::new().atleast_version("54.1").probe("libavutil").unwrap(),