aes: remove use of aarch64_target_feature

It's been stabilized: rust-lang/rust#90620

Because of that, it's breaking the build on recent nightlies:

https://github.com/RustCrypto/block-ciphers/runs/7968517726?check_suite_focus=true#step:7:103

> error: the feature `aarch64_target_feature` has been stable since
> 1.61.0 and no longer requires an attribute to enable

Author: tarcieri

Cargo.lock

@@ -120,10 +120,7 @@
 )]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 #![warn(missing_docs, rust_2018_idioms)]
-#![cfg_attr(
-    all(aes_armv8, target_arch = "aarch64"),
-    feature(stdsimd, aarch64_target_feature)
-)]
+#![cfg_attr(all(aes_armv8, target_arch = "aarch64"), feature(stdsimd))]
 
 #[cfg(feature = "hazmat")]
 pub mod hazmat;