refactor(lib): remove build script features since minimum version is higher

src/headers.rs

@@ -2,8 +2,6 @@ use bytes::BytesMut;
 use http::HeaderMap;
 use http::header::{CONTENT_LENGTH, TRANSFER_ENCODING};
 use http::header::{HeaderValue, OccupiedEntry, ValueIter};
-#[cfg(not(__hyper_inherent_ascii))]
-use std::ascii::AsciiExt;
 
 pub fn connection_keep_alive(value: &HeaderValue) -> bool {
     connection_has(value, "keep-alive")