Remove duplicate not done comment

As indicated in #259 , I found it confusing to have 2 comments as the code wouldn't compile unless I solved both issues (I used the script from #281 to remove a comment and use `:wn` to go to next exercise, hence this tripped me).

exercises/conversions/as_ref_mut.rs

@@ -2,7 +2,6 @@
 // Read more about them at https://doc.rust-lang.org/std/convert/trait.AsRef.html
 // and https://doc.rust-lang.org/std/convert/trait.AsMut.html, respectively.
 
-// I AM NOT DONE
 // Obtain the number of bytes (not characters) in the given argument
 // Add the AsRef trait appropriately as a trait bound
 fn byte_counter<T>(arg: T) -> usize {