survey: Remove Cython overlay for Python 2.

Nixpkgs uses Python 3 almost everywhere now, so it should no longer be
needed.

Author: nh2

survey/default.nix

@@ -1,16 +1,4 @@
 let
-  cython-disable-tests-overlay = final: previous: {
-    python27 = previous.python27.override {
-      packageOverrides = self: super: {
-        cython = super.cython.overridePythonAttrs (old: rec {
-          # TODO Remove once Cython tests are no longer flaky. See
-          #   https://github.com/nh2/static-haskell-nix/issues/6#issuecomment-420452838
-          #   https://github.com/cython/cython/issues/2602
-          doCheck = false;
-        });
-      };
-    };
-  };
 in
 
 {
@@ -31,7 +19,6 @@ in
   # Note that we must NOT use something like `import normalPkgs.path {}`.
   # It is bad because it removes previous overlays.
   pkgs ? (normalPkgs.appendOverlays [
-    cython-disable-tests-overlay
   ])."${approach}",
 
   # When changing this, also change the default version of Cabal declared below