Remove redundant extern crate calls from chapter 3 source.

chapter-03-walkmap/src/main.rs

@@ -1,6 +1,4 @@
-extern crate rltk;
 use rltk::{Console, GameState, Rltk, RGB, VirtualKeyCode};
-extern crate specs;
 use specs::prelude::*;
 #[macro_use]
 extern crate specs_derive;