Remove core_graphics::private::CGSRegion (#700)

This is a private API on macOS. This code was added for use
in `servo-glutin` and was not used outside of that as far as
I know (by doing various code searches).

Fixes #656.

Author: waywardmonkeys

core-graphics/src/lib.rs

@@ -25,8 +25,6 @@ pub mod geometry;
 pub mod gradient;
 pub mod image;
 pub mod path;
-#[cfg(target_os = "macos")]
-pub mod private;
 pub mod sys;
 #[cfg(target_os = "macos")]
 pub mod window;