Clean up and unify exercises

exercises/16_lifetimes/lifetimes2.rs

@@ -1,10 +1,5 @@
-// lifetimes2.rs
-//
 // So if the compiler is just validating the references passed to the annotated
 // parameters and the return type, what do we need to change?
-//
-// Execute `rustlings hint lifetimes2` or use the `hint` watch subcommand for a
-// hint.
 
 fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
     if x.len() > y.len() {