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.

solutions/09_strings/strings4.rs

@@ -15,15 +15,6 @@ fn main() {
 
     string("rust is fun!".to_owned());
 
-    // Here, both answers work.
-    // `.into()` converts a type into an expected type.
-    // If it is called where `String` is expected, it will convert `&str` to `String`.
-    string("nice weather".into());
-    // But if it is called where `&str` is expected, then `&str` is kept as `&str` since no conversion is needed.
-    // If you remove the `#[allow(…)]` line, then Clippy will tell you to remove `.into()` below since it is a useless conversion.
-    #[allow(clippy::useless_conversion)]
-    string_slice("nice weather".into());
-
     string(format!("Interpolation {}", "Station"));
 
     // WARNING: This is byte indexing, not character indexing.