Remove a bunch of now-unnecessary const_let feature gates

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

@@ -1,26 +0,0 @@
-// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// run-pass
-#![allow(dead_code)]
-
-// test that certain things are disallowed in constant functions
-
-#![feature(const_fn, const_let)]
-
-// no destructuring
-const fn i((
-            a,
-            b
-           ): (u32, u32)) -> u32 {
-    a + b
-}
-
-fn main() {}

src/test/run-pass/ctfe/issue-37550.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/ui/consts/const_let_assign2.rs

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

src/test/ui/issues/issue-44056.rs

@@ -1,16 +0,0 @@
-// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
-// compile-pass
-// only-x86_64
-// no-prefer-dynamic
-// compile-flags: -Ctarget-feature=+avx -Clto
-
-fn main() {}