It always returned wxDragNone since the changes of f5548e399e (Fix
problem with dragged icon remaining on screen under MSW 10, 2020-01-11)
which wrongly changed pdwEffect to DROPEFFECT_NONE before doing anything
else, disregarding the fact that it's an in-out parameter and not just
an output one.
Fix this by simply not doing this, just as it hadn't been done before.
See #18499.
b6c593af45