V4L/DVB (10870): v4l2-ioctl: get rid of video_decoder.h

The V4L1 obsoleted header video_decoder.h is not used anymore by any driver. Only
a name decoding function at v4l2-ioctl still implements it.

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