Remove feature annotation

This is not needed anymore.

src/main.rs

@@ -1,5 +1,3 @@
-#![feature(proc_macro_hygiene, decl_macro)]
-
 use anyhow::{anyhow, bail, Result};
 use clap::{crate_authors, crate_version, AppSettings, Parser};
 use hmac::{Hmac, Mac, NewMac};