NSIS/Qt: Move Qt version logic to windeployqt-to-nsis.

Move Qt version checking from packaging/nsis/Makefile.nmake to
windeployqt-to-nsis.ps1. Get rid of the -Windeployqt parameter since
its absence is a condition we now handle.

Change-Id: Ieba86d5a91e8217ed11dbe57e80bedaccd2e99cf
Reviewed-on: https://code.wireshark.org/review/3595
Reviewed-by: Gerald Combs <gerald@wireshark.org>