Remove redundant forward declaration of wxCursor from wx/window.h.

We include wx/cursor.h from wx/window.h anyhow (and can't avoid it as we have
a member of wxCursor type in wxWindowBase).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76165 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775