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/subcontext.h b/init/subcontext.h
index 628fd50..16bd870 100644
--- a/init/subcontext.h
+++ b/init/subcontext.h
@@ -42,7 +42,7 @@
         Fork();
     }
 
-    Result<Success> Execute(const std::vector<std::string>& args);
+    Result<void> Execute(const std::vector<std::string>& args);
     Result<std::vector<std::string>> ExpandArgs(const std::vector<std::string>& args);
     void Restart();