[media] media-entity.h: convert media_entity_cleanup to inline

This function was used in the past to free the links
that were allocated by the media controller core.

However, this is not needed anymore. We should likely
get rid of the funcion on some function, but, for now,
let's just convert into an inlined function and let the
compiler to get rid of it.

Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>