Remove use of pinfo->private_data since it's not used anywhere.

Change-Id: Ibb47fd1a0d498cc9791ca31ee625395905a7e999
Reviewed-on: https://code.wireshark.org/review/5464
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>