Remove doc_auto_cfg to clear errors on nightly

```
error[E0557]: feature has been removed
  --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/socket2-0.6.0/src/lib.rs:57:29
   |
57 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in CURRENT_RUSTC_VERSION; see <rust-lang/rust#138907> for more information
   = note: merged into `doc_cfg`
```

Author: bobrik

foundations/src/lib.rs

@@ -68,7 +68,6 @@
 // NOTE: required to allow cfgs like `tokio_unstable` on nightly which is used in tests.
 #![allow(unexpected_cfgs)]
 #![warn(missing_docs)]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
 mod utils;