distrib/configure.ac.in: remove mention to 'settings', since settings.in is gone

Otherwise, when ./configureing a GHC bindist, produced by either Make or Hadrian, we would try to generate the settings file from the settings.in template that we used to have around but which has been gone since d37d91e9. That commit generates the settings file using the build systems instead, but forgot to remove this mention to the settings file.

To be precise, I only ran into the problem and successfully solved it with this patch against a Hadrian bindist, I haven't tried with one produced by Make, but I'm pretty confident they both use distrib/configure.ac.in so I expect that the same error occurs there, effectively meaning that all our bindists are broken...?