Drop now-removed doc_auto_cfg feature

It seems new rustc removed this feature, hopefully its available
upstream but either way it caused the 0.1.6 docs.rs build to fail.

Backport of 68e4314

Author: TheBlueMatt

lightning/src/lib.rs

@@ -37,7 +37,6 @@
 // that don't do anything but annoy us and cant actually ever be resolved.
 #![allow(bare_trait_objects)]
 #![allow(ellipsis_inclusive_range_patterns)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
 
 #[cfg(all(fuzzing, test))]