xous: sync: remove rustc_const_stable attribute

These functions had `#[rustc_const_stable(feature = "const_locks", since
= "1.63.0")]` on them because they were originally taken from
`no_threads`. with d066dfd these no longer compile. Since other
platforms do not have this attribute, remove it. This fixes the build
for Xous.

Signed-off-by: Sean Cross <sean@xobs.io>

library/std/src/sys/sync/condvar/xous.rs

@@ -20,7 +20,6 @@ unsafe impl Sync for Condvar {}
 
 impl Condvar {
     #[inline]
-    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
     pub const fn new() -> Condvar {
         Condvar { counter: AtomicUsize::new(0), timed_out: AtomicUsize::new(0) }
     }

library/std/src/sys/sync/mutex/xous.rs

@@ -24,7 +24,6 @@ pub struct Mutex {
 
 impl Mutex {
     #[inline]
-    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
     pub const fn new() -> Mutex {
         Mutex { locked: AtomicUsize::new(0), contended: AtomicBool::new(false) }
     }