* Remove cabal.project.libonly

It was only referenced once in a stale Makefile comment about doctests.

* Remove weeder

- remove weeder's configuration
- remove its recipe from Makefile
- remove its project

* Delete cabal.project.doctest

- Adding --ghc-options="-Wwarn" is sufficient to avoid the numerous <interactive> failures seen otherwise
- write-ghc-environment-files has a default of never