Revert "Correctly detect cancelled drag-and-drop operations in wxGTK."

This reverts r75745 (and r75749 fixing a minor problem in it) as this change
broke the reporting of the result of the drag and drop operation instead of
fixing it.

See #15930.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76649 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775