[media] cx18: remove g_chip_ident support

The av-core is really a subdev, so there is no need anymore to act as if it
is a 'second' bridge chip.
As a result of this the g_chip_ident implementation can be completely dropped.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>