Remove two-argument version of inferReaderDriver

Per discussion on 01c077cca37687e2c159796011de44fcc9dda068, the
two-argument version of StageFactory::inferReaderDriver does a bit too
much. This patch removes the two argument version, and updates all uses
to the new, one-argument version.

This does not change inferWriterDriver. That method does some meaningful
tweaking to the passed-in options, so its removal should be a seperate
commit (+pull request) IMO.