update_engine: Remove release/aquire semantic from scoped_ptr/unique_ptr

Now that scoped_ptr is just a type alias to unique_ptr, there is no
longer a need to convert between the two by using .release() and
constructing the other with a raw pointer.

BUG: None
TEST: Built for dragonboard, all unit tests pass (except for update_engine,
      but those tests failed before these changes too).

Change-Id: I0afcbc4d85b7ca78149fb29c188d7bcc99d4c80b
diff --git a/update_manager/evaluation_context.cc b/update_manager/evaluation_context.cc
index 03ac0b7..63f7d9b 100644
--- a/update_manager/evaluation_context.cc
+++ b/update_manager/evaluation_context.cc
@@ -92,7 +92,7 @@
   MessageLoop::current()->CancelTask(timeout_event_);
   timeout_event_ = MessageLoop::kTaskIdNull;
 
-  return unique_ptr<Closure>(callback_.release());
+  return std::move(callback_);
 }
 
 TimeDelta EvaluationContext::RemainingTime(Time monotonic_deadline) const {