Delete deprecated AsciiExt extension trait import

This has been superseded by inherent methods since Rust 1.26.

serde_derive/src/internals/case.rs

@@ -1,10 +1,6 @@
 //! Code to convert the Rust-styled field/variant (e.g. `my_field`, `MyType`) to the
 //! case of the source (e.g. `my-field`, `MY_FIELD`).
 
-// See https://users.rust-lang.org/t/psa-dealing-with-warning-unused-import-std-ascii-asciiext-in-today-s-nightly/13726
-#[allow(deprecated, unused_imports)]
-use std::ascii::AsciiExt;
-
 use std::fmt::{self, Debug, Display};
 
 use self::RenameRule::*;