Remove a bunch of now-unnecessary const_let feature gates

Author: oli-obk

src/libcore/lib.rs

@@ -120,7 +120,7 @@
 #![feature(const_slice_len)]
 #![feature(const_str_as_bytes)]
 #![feature(const_str_len)]
-#![feature(const_let)]
+#![cfg_attr(stage0, feature(const_let))]
 #![feature(const_int_rotate)]
 #![feature(const_int_wrapping)]
 #![feature(const_int_sign)]

src/test/run-pass/ctfe/const-fn-destructuring-arg.rs

@@ -12,7 +12,7 @@
 #![allow(dead_code)]
 #![allow(unused_variables)]
 
-#![feature(const_fn, const_let)]
+#![feature(const_fn)]
 
 const fn x() {
     let t = true;

src/test/run-pass/ctfe/issue-37550.rs

@@ -1,8 +1,5 @@
 // compile-pass
 
-#![feature(const_let)]
-#![feature(const_fn)]
-
 pub struct AA {
     pub data: [u8; 10],
 }