Clean up and unify exercises

exercises/15_traits/traits4.rs

@@ -1,11 +1,6 @@
-// traits4.rs
-//
 // Your task is to replace the '??' sections so the code compiles.
 //
 // Don't change any line other than the marked one.
-//
-// Execute `rustlings hint traits4` or use the `hint` watch subcommand for a
-// hint.
 
 pub trait Licensed {
     fn licensing_info(&self) -> String {