python3Packages.confluent-kafka: drop obsolete httpx fix

pkgs/development/python-modules/confluent-kafka/default.nix

···
     respx
   ] ++ lib.flatten (lib.attrValues optional-dependencies);
 
-  preCheck = ''
-    # httpx since 0.28.0+ depends on SSL_CERT_FILE
-    SSL_CERT_FILE=${cacert}/etc/ssl/certs/ca-bundle.crt
-  '';
-
   pythonImportsCheck = [ "confluent_kafka" ];
 
   disabledTestPaths = [