Revert "Plug memory leak in fibers"

This reverts commit 88bd8df.

The reason why this looked like a leak is because we clean up fiber data
memory in the fiber finalizer. We don't run those yet on exit properly,
so that's why it looked like a leak but actually isn't.

vm/fiber_stack.cpp

@@ -57,9 +57,7 @@ namespace rubinius {
         i != datas_.end();
         ++i)
     {
-      FiberData* data = *i;
-      data->die();
-      delete data;
+      (*i)->die();
     }
 
     for(Stacks::iterator i = stacks_.begin();