remove man-cache rebuild

it isn't really needed in xonsh.
Just the autocompletion is broken

modules/xonsh/default.nix

@@ -15,9 +15,5 @@ in
   users.users.${username} = {
     shell = pkgs.xonsh;
   };
-  documentation.man.generateCaches = true;
-  home-manager.users.${username} = {
-    programs.man.generateCaches = true;
-  };
 }