Use LocatePythonModule to locate asn2wrs.py

This reuses existing functionality and thereby making FindAsn2Wrs.cmake obsolete, resulting in a reduction of the cmake module maintaince load

Change-Id: Ic6ae53e65236246c0cbe86f8c2f066a2d034e0b9
Reviewed-on: https://code.wireshark.org/review/5805
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>