No longer put windows condvars in a box.

Windows condition variables are movable (while not borrowed) according
to their documentation.

library/std/src/sys/windows/condvar.rs

@@ -8,7 +8,7 @@ pub struct Condvar {
     inner: UnsafeCell<c::CONDITION_VARIABLE>,
 }
 
-pub type MovableCondvar = Box<Condvar>;
+pub type MovableCondvar = Condvar;
 
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}