python3Packages.caldav: remove outdated substituteInPlace

https://github.com/python-caldav/caldav/commit/1abfac52725c5d79d5d954f4c3b8e61381604e3b
https://github.com/python-caldav/caldav/commit/63603d91a092e2deac8919459f8c06ccd5f88187

This is also safe to do since it does not test xandikos or radicale by default
if they are not available

https://github.com/python-caldav/caldav/blob/f7e00211d6230c4c2ec84645ebce6f1140394fb7/tests/conf.py#L49

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

@@ -45,13 +45,6 @@ buildPythonPackage rec {
     pytestCheckHook
   ];
 
-  # xandikos and radicale are only optional test dependencies, not available for python3
-  postPatch = ''
-    substituteInPlace setup.py \
-      --replace xandikos "" \
-      --replace radicale ""
-  '';
-
   pythonImportsCheck = [ "caldav" ];
 
   meta = with lib; {