python3.pkgs.dask: don't offer an option for extras-require

Using such an option within the package set this way will result in
different packages depending on different versions of dask and thereby
collisions.

Instead, we can simply declare in the package offering the extras the
packages the extra needs.

pkgs/development/python-modules/datashader/default.nix

@@ -46,7 +46,7 @@ buildPythonPackage rec {
     param
     pyct
     scipy
-  ];
+  ] ++ dask.extras-require.complete;
 
   checkInputs = [
     pytestCheckHook