opcua: remove unused code

Change-Id: I78963640e32393f3ac399e59a659ddf619d15c3d
Reviewed-on: https://code.wireshark.org/review/4148
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit d8afd59b554e30c0063faa48b98703065c2057a3)
Reviewed-on: https://code.wireshark.org/review/4240