remove hint comments when no hint exists

exercises/clippy/clippy3.rs

@@ -1,8 +1,7 @@
 // clippy3.rs
 // 
 // Here's a couple more easy Clippy fixes, so you can see its utility.
-//
-// Execute `rustlings hint clippy3` or use the `hint` watch subcommand for a hint.
+// No hints.
 
 // I AM NOT DONE