into string should not modify strings (#15320)

# Description

`into string` should not modify input strings (even with the
`--group-digits` flag). It's a conversion command, not a formatting
command.

# User-Facing Changes

- For strings, the same behavior from 0.102.0 is preserved.
- Errors are no longer turned into strings, but rather they are returned
as is.

# After Submitting

Create a `format int` and/or `format float` command and so that the
`--group-digits` flag can be transferred to one of those commands.

crates/nu-protocol/src/errors/shell_error/mod.rs

@@ -1474,10 +1474,6 @@ impl<'de> Deserialize<'de> for ShellError {
     }
 }
 
-pub fn into_code(err: &ShellError) -> Option<String> {
-    err.code().map(|code| code.to_string())
-}
-
 #[test]
 fn shell_error_serialize_roundtrip() {
     // Ensure that we can serialize and deserialize `ShellError`, and check that it basically would