lib: remove the provide_any feature when backtrace is activated

It would seem the provide_any feature has been removed from nightly
rust[1], so this does not compile anymore. Fix this by removing this
attribute.

[1] rust-lang/rust#96024 (comment)

Author: tshikaboom

src/lib.rs

@@ -3,7 +3,6 @@
 #![warn(missing_docs)]
 #![deny(unused_must_use)]
 #![cfg_attr(feature = "backtrace", feature(error_generic_member_access))]
-#![cfg_attr(feature = "backtrace", feature(provide_any))]
 
 pub mod errors;
 mod fsshttpb;