Revert "Add Colorable supertrait"

This reverts commit cbd4c4b.

Since the trait needs to be in scope anyway it does not help with
reducing the number of necessary imports; the only thing that would help
is merging the `Background` and `Foreground` traits into a single one.

src/lib.rs

@@ -80,8 +80,4 @@ mod foreground;
 
 pub use background::*;
 pub use colors::*;
-pub use foreground::*;
-
-pub trait Colorable: Background + Foreground {}
-
-impl<T: Background + Foreground> Colorable for T {}
+pub use foreground::*;