Clean up and unify exercises

exercises/09_strings/strings1.rs

@@ -1,9 +1,4 @@
-// strings1.rs
-//
 // Make me compile without changing the function signature!
-//
-// Execute `rustlings hint strings1` or use the `hint` watch subcommand for a
-// hint.
 
 fn main() {
     let answer = current_favorite_color();

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 :)

exercises/09_strings/strings3.rs

@@ -1,8 +1,3 @@
-// strings3.rs
-//
-// Execute `rustlings hint strings3` or use the `hint` watch subcommand for a
-// hint.
-
 fn trim_me(input: &str) -> String {
     // TODO: Remove whitespace from both ends of a string!
     ???

exercises/09_strings/strings4.rs

@@ -1,11 +1,7 @@
-// strings4.rs
-//
-// Ok, here are a bunch of values-- some are `String`s, some are `&str`s. Your
+// Ok, here are a bunch of values - some are `String`s, some are `&str`s. Your
 // task is to call one of these two functions on each value depending on what
 // you think each value is. That is, add either `string_slice` or `string`
 // before the parentheses on each line. If you're right, it will compile!
-//
-// No hints this time!
 
 fn string_slice(arg: &str) {
     println!("{}", arg);