Remove doc_auto_cfg attribute

This attribute has been removed from the nightly toolchain. It breaks
the nightly build in CI and also if/when we release it breaks the docs
build on crates.io which uses a nightly toolchain.

In `rust-bitcoin` we elected to just remove it instead of replacing it
as suggested by many in the wider eccosystem because we figured they
broke us once better not to rely on them again.

Author: tcharding

src/lib.rs

@@ -131,7 +131,6 @@
 #![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
 // Experimental features we need.
 #![cfg_attr(bench, feature(test))]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 // Coding conventions
 #![deny(missing_docs)]
 #![allow(clippy::suspicious_arithmetic_impl)] // this lint is literally always wrong