Clean up and unify exercises

exercises/09_strings/strings2.rs

@@ -1,9 +1,4 @@
-// strings2.rs
-//
 // Make me compile without changing the function signature!
-//
-// Execute `rustlings hint strings2` or use the `hint` watch subcommand for a
-// hint.
 
 fn main() {
     let word = String::from("green"); // Try not changing this line :)