Remove useless 'static bounds on Box allocator

#79327 added `'static` bounds to the allocator parameter
for various `Box` + `Pin` APIs to ensure soundness.
But it was a bit overzealous, some of the bounds aren't
actually needed.

library/alloc/src/boxed.rs

@@ -2104,13 +2104,10 @@ impl<T: ?Sized, A: Allocator> AsMut<T> for Box<T, A> {
  *  could have a method to project a Pin<T> from it.
  */
 #[stable(feature = "pin", since = "1.33.0")]
-impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> where A: 'static {}
+impl<T: ?Sized, A: Allocator> Unpin for Box<T, A> {}
 
 #[unstable(feature = "coroutine_trait", issue = "43122")]
-impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A>
-where
-    A: 'static,
-{
+impl<G: ?Sized + Coroutine<R> + Unpin, R, A: Allocator> Coroutine<R> for Box<G, A> {
     type Yield = G::Yield;
     type Return = G::Return;
 
@@ -2133,10 +2130,7 @@ where
 }
 
 #[stable(feature = "futures_api", since = "1.36.0")]
-impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A>
-where
-    A: 'static,
-{
+impl<F: ?Sized + Future + Unpin, A: Allocator> Future for Box<F, A> {
     type Output = F::Output;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {