scikit-learn: remove unused environment variable in build script (#325522)

pkgs/development/python-modules/scikit-learn/default.nix

···
 
   env.LC_ALL = "en_US.UTF-8";
 
-  preBuild = ''
-    export SKLEARN_BUILD_PARALLEL=$NIX_BUILD_CORES
-  '';
-
   # PermissionError: [Errno 1] Operation not permitted: '/nix/nix-installer'
   doCheck = !stdenv.hostPlatform.isDarwin;