Elm cnp: Fix infinite loop on drop target deletions in some cases.

Elm cnp: Fix infinite loop on drop target deletions in some cases.

In some cases, like having a drop target inside an inwin (looks like it
can be other containers too) can cause an infinite loop (as described in
the report). The reason for that is that while the drop target was added
when there was an X window available, the X window ws now gone, so the
non X path was being called which didn't have the code to remove the
item from the list being iterated. Yes, definition of spaghetti and
false assumptions.
Elm dnd/cnp need a massive overhaul, they are disgusting.

See the ticket for more information on the issue.

Fixes T1702