Remove tz Haskell package override from cabal2nix

We've decided that the entire tz override doesn't make sense in
cabal2nix.

The tests for the tz Haskell package require the tzdata system library,
but we can add that in Nixpkgs.  That seems like the least confusing way
to provide it.

cabal2nix/src/Distribution/Nixpkgs/Haskell/FromCabal/PostProcess.hs

@@ -174,7 +174,6 @@ hooks =
   , ("tensorflow-proto", set (libraryDepends . tool . contains (pkg "protobuf")) True)
   , ("thyme", set (libraryDepends . tool . contains (self "cpphs")) True) -- required on Darwin
   , ("twilio", set doCheck False)         -- attempts to access the network
-  , ("tz", tzOverrides)
   , ("udev", set (metaSection . platforms) (Just $ Set.singleton (NixpkgsPlatformGroup (ident # "linux"))))
   , ("websockets", set doCheck False)   -- https://github.com/jaspervdj/websockets/issues/104
   , ("Win32", set (metaSection . platforms) (Just $ Set.singleton (NixpkgsPlatformGroup (ident # "windows"))))
@@ -405,12 +404,5 @@ bustleOverrides = set (libraryDepends . pkgconfig . contains "system-glib = pkgs
                 . set (metaSection . license) (Known "lib.licenses.lgpl21Plus")
                 . set (metaSection . hydraPlatforms) Nothing
 
--- | The tz Haskell package depends on both the tzdata Haskell package, as well as the
--- tzdata system package.  The following passes in the tzdata system package as
--- \"system-tzdata\".
-tzOverrides :: Derivation -> Derivation
-tzOverrides =
-  set (testDepends . system . contains "system-tzdata = pkgs.tzdata") True
-
 nullBinding :: Identifier -> Binding
 nullBinding name = binding # (name, path # [ident # "null"])