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

Otherwise, when `./configure`ing 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.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc