Clean up and unify exercises

exercises/16_lifetimes/lifetimes1.rs

@@ -1,12 +1,7 @@
-// lifetimes1.rs
-//
 // The Rust compiler needs to know how to check whether supplied references are
 // valid, so that it can let the programmer know if a reference is at risk of
 // going out of scope before it is used. Remember, references are borrows and do
 // not own their own data. What if their owner goes out of scope?
-//
-// Execute `rustlings hint lifetimes1` or use the `hint` watch subcommand for a
-// hint.
 
 fn longest(x: &str, y: &str) -> &str {
     if x.len() > y.len() {