init: replace Result<Success> with Result<void>

Now that Result<T> is actually expected<T, ...>, and the expected
proposal states expected<void, ...> as the way to indicate an expected
object that returns either successfully with no object or an error,
let's move init's Result<Success> to the preferred Result<void>.

Bug: 132145659
Test: boot, init unit tests
Change-Id: Ib2f98396d8e6e274f95a496fcdfd8341f77585ee
diff --git a/init/reboot.cpp b/init/reboot.cpp
index 54f68bb..fbc03c2 100644
--- a/init/reboot.cpp
+++ b/init/reboot.cpp
@@ -707,7 +707,7 @@
     // Queue built-in shutdown_done
     auto shutdown_handler = [cmd, command, reboot_target, run_fsck](const BuiltinArguments&) {
         DoReboot(cmd, command, reboot_target, run_fsck);
-        return Success();
+        return Result<void>{};
     };
     ActionManager::GetInstance().QueueBuiltinAction(shutdown_handler, "shutdown_done");