std: Remove UnsafeArc

This type has been superseded by Arc<Unsafe<T>>. The UnsafeArc type is a relic
of an era that has long since past, and with the introduction of liballoc the
standard library is able to use the Arc smart pointer. With little need left for
UnsafeArc, it was removed.

All existing code using UnsafeArc should either be reevaluated to whether it can
use only Arc, or it should transition to Arc<Unsafe<T>>

[breaking-change]

src/libstd/sync/mpsc_queue.rs

@@ -165,7 +165,7 @@ mod tests {
 
     #[test]
     fn test_full() {
-        let mut q = Queue::new();
+        let q = Queue::new();
         q.push(box 1);
         q.push(box 2);
     }
@@ -174,7 +174,7 @@ mod tests {
     fn test() {
         let nthreads = 8u;
         let nmsgs = 1000u;
-        let mut q = Queue::new();
+        let q = Queue::new();
         match q.pop() {
             Empty => {}
             Inconsistent | Data(..) => fail!()