[media] media-entity.h: remove redundant macro definition for gobj_to_link()

The macro gobj_to_link() is defined twice in media-entity.h.
Let's remove one.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>