MetaWaylandDataDevice: Don't set surface offset as anchor offset

Since we are using the surface actor to draw the DND icon, the offset
is already accounted for by MetaSurfaceActorWayland, and passing the
surface position offset would effectively double the actual offset,
causing the icon to be misplaced.

This patch always sets the anchor offset to (0, 0) when the icon is a
Wayland surface, and lets the surface actor deal with the offsetting.

https://bugzilla.gnome.org/show_bug.cgi?id=759222

src/wayland/meta-wayland-surface.c

@@ -257,8 +257,6 @@ dnd_surface_commit (MetaWaylandSurfaceRole  *surface_role,
     meta_wayland_surface_role_get_surface (surface_role);
 
   meta_wayland_surface_queue_pending_state_frame_callbacks (surface, pending);
-
-  meta_wayland_data_device_update_dnd_surface (&surface->compositor->seat->data_device);
 }
 
 static void