Remove private_data member from packet_info structure.

Dissectors should pass data directly to their subdissectors through the data parameter (of new-style dissectors). This avoids unintentional "trampling" from other dissectors trying to "share" private_data member.

Change-Id: I2efef5c8dfeef64588ba3ac6e695b469238c6468
Reviewed-on: https://code.wireshark.org/review/5487
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Mann <mmann78@netscape.net>