strings4: remove From-based conversion

To understand From-based conversion, an understanding of traits is
required, which we teach in a later chapter. The From trait specifically
is taught in one of the conversion exercises. So, we can safely remove
it here without users missing out on learning something important.

A specific source of confusion for users was a warning that the
conversion is useless, which appeared when using the `string_slice`
function for the expression with `.into()`. closes #2190

exercises/09_strings/strings4.rs

@@ -21,8 +21,6 @@ fn main() {
 
     placeholder("rust is fun!".to_owned());
 
-    placeholder("nice weather".into());
-
     placeholder(format!("Interpolation {}", "Station"));
 
     // WARNING: This is byte indexing, not character indexing.