Remove doc_auto_cfg

Attribute has been removed in recent nightly toolchain and this breaks
the docs build on docsrs.

Author: tcharding

secp256k1-sys/src/lib.rs

@@ -7,7 +7,6 @@
 // Coding conventions
 #![deny(non_upper_case_globals, non_camel_case_types, non_snake_case, unused_mut)]
 #![cfg_attr(all(not(test), not(feature = "std")), no_std)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
 #[cfg(any(test, feature = "std"))]
 extern crate core;

src/lib.rs

@@ -149,7 +149,6 @@
 #![warn(missing_docs, missing_copy_implementations, missing_debug_implementations)]
 #![cfg_attr(all(not(test), not(feature = "std")), no_std)]
 // Experimental features we need.
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![cfg_attr(bench, feature(test))]
 
 #[cfg(feature = "alloc")]