Remove doc_auto_cfg

[gwenn]

merge into doc_cfg

[gwenn]

See https://docs.rs/crate/rustyline/17.0.2/builds/2557460

[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]   --> src/lib.rs:20:29
[INFO] [stderr]    |
[INFO] [stderr] 20 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]    |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]    |
[INFO] [stderr]    = note: removed in CURRENT_RUSTC_VERSION; see <https://github.com/rust-lang/rust/pull/138907> for more information
[INFO] [stderr]    = note: merged into `doc_cfg`