Remove allow(unknown_lints)

We do not have any unknown lints, I'm not even sure its possible to get
them now that we have the MSRV in the clippy config file.

Author: tcharding

src/lib.rs

@@ -29,8 +29,7 @@ assert_eq!(variant, Variant::Bech32);
 )]
 //!
 
-// Allow trait objects without dyn on nightly and make 1.22 ignore the unknown lint
-#![allow(unknown_lints)]
+// Allow trait objects without dyn on nightly.
 #![allow(bare_trait_objects)]
 #![deny(missing_docs)]
 #![deny(non_upper_case_globals)]