Revert "cabal: enable multi-threaded builds with GHC 7.8.x or later"

This likely exacerbates the non-determinism in ghc package ids, so until
that is fixed let's live with the slow builds.

This reverts commit 817c0e41443a5176baf6dd9b422878fdccecd266.
Merge pull request #4554 from shlevy/no-parallel-haskell

Revert "cabal: enable multi-threaded builds with GHC 7.8.x or later"