remove parent parameter from try_dissect_next_protocol()

Change-Id: Ib05b67c8741f90444e773e7c9e1378f00f41dad7
Reviewed-on: https://code.wireshark.org/review/5083
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>