std: Stabilize the thread_local_const_init feature

This commit is intended to follow the stabilization disposition of the
FCP that has now finished in #84223. This stabilizes the ability to flag
thread local initializers as `const` expressions which enables the macro
to generate more efficient code for accessing it, notably removing
runtime checks for initialization.

More information can also be found in #84223 as well as the tests where
the feature usage was removed in this PR.

Closes #84223

compiler/rustc_span/src/lib.rs

@@ -20,7 +20,6 @@
 #![feature(negative_impls)]
 #![feature(nll)]
 #![feature(min_specialization)]
-#![feature(thread_local_const_init)]
 
 #[macro_use]
 extern crate rustc_macros;