Remove `AsciiExt` import (#104)

Since the methods on this trait were moved to direct implementations on the
implementing types, this produces an unused import warning with the latest
(1.23) Rust standard library. As we set `deny(warnings)`, this breaks the build.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

proxy/src/fully_qualified_authority.rs

@@ -1,6 +1,5 @@
 use bytes::BytesMut;
 
-use std::ascii::AsciiExt;
 use std::net::IpAddr;
 use std::str::FromStr;