kbfs: avoid using PrivateTmp for systemd service

The kbfs service does not seem to actually work with a PrivateTmp. A
PrivateTmp seems as appropriate for kbfs as anything else, but the
upstream service does not use it either
<https://github.com/keybase/client/blob/master/packaging/linux/systemd/kbfs.service>

modules/services/kbfs.nix

@@ -52,7 +52,6 @@ in {
           "${pkgs.kbfs}/bin/kbfsfuse ${toString cfg.extraFlags} ${mountPoint}";
         ExecStopPost = "/run/wrappers/bin/fusermount -u ${mountPoint}";
         Restart = "on-failure";
-        PrivateTmp = true;
       };
 
       Install.WantedBy = [ "default.target" ];