Clean up and unify exercises

exercises/23_conversions/try_from_into.rs

@@ -1,13 +1,8 @@
-// try_from_into.rs
-//
 // TryFrom is a simple and safe type conversion that may fail in a controlled
 // way under some circumstances. Basically, this is the same as From. The main
 // difference is that this should return a Result type instead of the target
 // type itself. You can read more about it at
 // https://doc.rust-lang.org/std/convert/trait.TryFrom.html
-//
-// Execute `rustlings hint try_from_into` or use the `hint` watch subcommand for
-// a hint.
 
 use std::convert::{TryFrom, TryInto};