Clean up and unify exercises

exercises/18_iterators/iterators4.rs

@@ -1,8 +1,3 @@
-// iterators4.rs
-//
-// Execute `rustlings hint iterators4` or use the `hint` watch subcommand for a
-// hint.
-
 pub fn factorial(num: u64) -> u64 {
     // Complete this function to return the factorial of num
     // Do not use:
@@ -12,7 +7,6 @@ pub fn factorial(num: u64) -> u64 {
     // - additional variables
     // For an extra challenge, don't use:
     // - recursion
-    // Execute `rustlings hint iterators4` for hints.
 }
 
 fn main() {