Revert "pretty: new haskell package"

This reverts commit 51dc75b7692ce41527685507ea7f6f5dc2cb6ec5. The pretty
library is a core package since (at least) GHC 6.10.4, i.e. every single one of
our compilers ships this library by default. Adding the package to the global
namespace like this is almost certainly going to cause trouble because we'll
end up with inconsistently linked binaries whenever packages mix the user-space
pretty library with the core one.