remove these two unneeded extern crate lines

i'm going to guess this was added early on automatically by clion
and i just never caught it. they'd been here since the project was
first committed to a git repo. oh well.

ggdt/src/lib.rs

@@ -1,6 +1,3 @@
-extern crate core;
-extern crate sdl2;
-
 pub mod audio;
 pub mod base;
 pub mod entities;