remove bitrotted cant_nest field from RWARC (the #[mutable] tag suffices)

Author: bblum

src/libextra/arc.rs

@@ -281,7 +281,6 @@ struct RWARCInner<T> { lock: RWlock, failed: bool, data: T }
 #[mutable]
 struct RWARC<T> {
     x: UnsafeAtomicRcBox<RWARCInner<T>>,
-    cant_nest: ()
 }
 
 /// Create a reader/writer ARC with the supplied data.
@@ -299,15 +298,14 @@ pub fn rw_arc_with_condvars<T:Const + Owned>(
     let data =
         RWARCInner { lock: rwlock_with_condvars(num_condvars),
                      failed: false, data: user_data };
-    RWARC { x: UnsafeAtomicRcBox::new(data), cant_nest: () }
+    RWARC { x: UnsafeAtomicRcBox::new(data), }
 }
 
 impl<T:Const + Owned> RWARC<T> {
     /// Duplicate a rwlock-protected ARC, as arc::clone.
     pub fn clone(&self) -> RWARC<T> {
         RWARC {
             x: self.x.clone(),
-            cant_nest: (),
         }
     }