Revert "Haskell: free-4.12.4 for ghc-7.10.x"

This reverts commit e419e98a24c2bb7458dd630bad0c224bab468351. It turns out the
older version is no longer required after all.

pkgs/development/haskell-modules/configuration-hackage2nix.yaml

···
   - containers < 0.5                    # required to build alex with GHC 6.12.3
   - control-monad-free < 0.6            # newer versions don't compile with anything but GHC 7.8.x
   - deepseq == 1.3.0.1                  # required to build Cabal with GHC 6.12.3
-  - free == 4.12.4                      # required on GHC 7.10.x
   - generic-deriving == 1.10.5.*        # new versions don't compile with GHC 7.10.x
   - gloss < 1.9.3                       # new versions don't compile with GHC 7.8.x
   - haddock < 2.17                      # required on GHC 7.10.x