Make rustc_next_trait_solver nightly again

compiler/rustc_next_trait_solver/src/lib.rs

@@ -4,8 +4,6 @@
 //! but were uplifted in the process of making the new trait solver generic.
 //! So if you got to this crate from the old solver, it's totally normal.
 
-#![feature(let_chains)]
-
 pub mod canonicalizer;
 pub mod infcx;
 pub mod resolve;