Don't accept data in unsupported format in wxMSW dnd code.

We wrongly pretended to accept the data in formats which we didn't actually
accept and showed misleading cursors to the user.

Fix this by partially reverting some of the changes of r72668 (see #14697).

Closes #16042.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76076 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775