Rollup merge of rust-lang#126737 - fee1-dead-contrib:rm-const-closures, r=compiler-errors

Remove `feature(const_closures)` from libcore

This is an incomplete feature and apparently it has no uses in `core`. Incomplete features should generally not be used in our standard library.

Author: matthiaskrgr

core/src/lib.rs

@@ -209,7 +209,6 @@
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_has_atomic_equal_alignment)]
-#![feature(const_closures)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_for)]
 #![feature(const_mut_refs)]