NSIS: Start winding down the Wireshark 2 Preview.

Users wishing to try the Qt UI should use the 1.99.x installers.

Change-Id: I97bbd049a8da1db63429d236b8afb33ab05ce8f7
Reviewed-on: https://code.wireshark.org/review/4914
Reviewed-by: Gerald Combs <gerald@wireshark.org>