haskell-hans: revert commit 983ab7be

https://github.com/NixOS/nixpkgs/pull/17555 added an override that pins the
version of haskellPackages.hans to a specific Git revision. This means that any
version updates in the Haskell packages set will have no effect and that our
users will be stuck at that (arbitrary) version until the override is removed
again. Furthermore, it's bad practice to distribute a package dubbed as version
X when it's really version Y! The proper way to fix this issue IMHO is to
convince upstream to released a fixed version to Hackage.