Remove Asynchronous Attribute

In the 0.8 release, we added AsynchronousAttribute and attempted to provide for asynchronous processing in pre-4.5 builds using that attribute and custom code. Further examination indicates that the implementation doesn't really do the job, since it relies on the creation of an extra thread to run the test. We're better served by implementing async/await on those platforms that support it and not supporting the async feature on lower-level builds.