Clean up and unify exercises

exercises/23_conversions/using_as.rs

@@ -1,14 +1,9 @@
-// using_as.rs
-//
 // Type casting in Rust is done via the usage of the `as` operator. Please note
 // that the `as` operator is not only used when type casting. It also helps with
 // renaming imports.
 //
 // The goal is to make sure that the division does not fail to compile and
 // returns the proper type.
-//
-// Execute `rustlings hint using_as` or use the `hint` watch subcommand for a
-// hint.
 
 fn average(values: &[f64]) -> f64 {
     let total = values.iter().sum::<f64>();