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/boot_control_chromeos.cc b/boot_control_chromeos.cc
index 547e72b..e9ad698 100644
--- a/boot_control_chromeos.cc
+++ b/boot_control_chromeos.cc
@@ -81,7 +81,7 @@
   if (!boot_control_chromeos->Init()) {
     LOG(ERROR) << "Ignoring BootControlChromeOS failure. We won't run updates.";
   }
-  return brillo::make_unique_ptr(boot_control_chromeos.release());
+  return std::move(boot_control_chromeos);
 }
 
 }  // namespace boot_control