media: media.h: remove __NEED_MEDIA_LEGACY_API

The __NEED_MEDIA_LEGACY_API define is 1) ugly and 2) dangerous
since it is all too easy for drivers to define it to get hold of
legacy defines. Instead just define what we need in media-device.c
which is the only place where we need the legacy define
(MEDIA_ENT_T_DEVNODE_UNKNOWN).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>