Remove rustc_allow_const_fn_ptr

This was a hack to work around the lack of an escape hatch for the "min
`const fn`" checks in const-stable functions. Now that we have co-opted
`allow_internal_unstable` for this purpose, we no longer need the
bespoke attribute.

Author: ecstatic-morse

core/src/task/wake.rs

@@ -129,14 +129,9 @@ impl RawWakerVTable {
     /// associated task.
     #[rustc_promotable]
     #[stable(feature = "futures_api", since = "1.36.0")]
-    // `rustc_allow_const_fn_ptr` is a hack that should not be used anywhere else
-    // without first consulting with T-Lang.
-    //
-    // FIXME: remove whenever we have a stable way to accept fn pointers from const fn
-    // (see https://github.com/rust-rfcs/const-eval/issues/19#issuecomment-472799062)
-    #[rustc_allow_const_fn_ptr]
     #[rustc_const_stable(feature = "futures_api", since = "1.36.0")]
     #[cfg_attr(not(bootstrap), allow_internal_unstable(const_fn_fn_ptr_basics))]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_ptr)]
     pub const fn new(
         clone: unsafe fn(*const ()) -> RawWaker,
         wake: unsafe fn(*const ()),