Remove classpath scanning in automagic batched hydration

[senior]

This code is pretty slow with large classpaths, taking over 10 seconds in Metabase. This code is also broken on JDK 9 due to the classloader changes, classpath/classpath will always return the empty list. With this removed, all of the Metabase tests still pass and we can save that 10 second load time the first time hydration is called.

[senior]

@camsaul This code didn't work at all on JDK 9 and our tests were still passing with it on Metabase. I removed it and tests pass with JDK 8 as well. The first thing that hits this code is the /api/user/current route, which takes a little over 10 seconds, after that initial load the results get cached and it doesn't call it again, but it's still noticeable on that first request.

[camsaul]

Yeah we can probably go ahead and take this out and speed things up a little for people. I think the docs say that Toucan automatically loads model namespaces somewhere so we need to find and update those docs too.