[media] dvb core: remove support for post FE legacy ioctl intercept

This DVB_FE_IOCTL_POST isn't used, so remove it.

Also, intercepting ioctl's like that only works with legacy ioctl's,
due to the way it was implemented. so this design is broken.
Document it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>