Delete orphan autoloading fixtures

Rails now delegates autoloading to Zeitwerk, and therefore does not need to test
autoloading itself. Zeitwerk has test coverage, in Rails we only need to test
the integration.

We are gradually trimming AS::Dependencies, and the AS test suite. With the
removal of DependenciesTestHelpers and client code in af27a25, these fixtures
became orphan.

Three of them are left. They are to be autoloaded with Module#autoload because
they raise errors when the file is evaluated. Their current use cases are
already committed.

Author: fxn