GDK W32: Don't check dest_surface for != NULL on button events

dest_surface is going to always be NULL for source contexts.
Previously we used to put the root window there to pass this check,
but root windows are gone (and root surfaces never existed to begin
with), so we have to adapt.

https://bugzilla.gnome.org/show_bug.cgi?id=773299

gdk/win32/gdkdnd-win32.c

@@ -3099,8 +3099,7 @@ gdk_dnd_handle_button_event (GdkDragContext       *context,
     return FALSE;
 #endif
 
-  if ((gdk_drag_context_get_selected_action (context) != 0) &&
-      (gdk_drag_context_get_dest_surface (context) != NULL))
+  if ((gdk_drag_context_get_selected_action (context) != 0))
     {
       g_signal_emit_by_name (context, "drop-performed",
                              gdk_event_get_time ((GdkEvent *) event));