wayland: Avoid redundant cancel() call

This is unnecessary as we are unsetting the DnD selection owner,
and will result in the related data source being cancelled already.

https://gitlab.gnome.org/GNOME/mutter/issues/591

(cherry picked from commit e7b2f9603e)

src/wayland/meta-wayland-data-device.c

@@ -1059,7 +1059,6 @@ drag_grab_button (MetaWaylandPointerGrab *grab,
         }
       else
         {
-          meta_wayland_data_source_cancel (source);
           meta_wayland_data_source_set_current_offer (source, NULL);
           meta_wayland_data_device_set_dnd_source (&seat->data_device, NULL);
           unset_selection_source (&seat->data_device, META_SELECTION_DND);