V4L/DVB (6587): Cleanup at tv norm selection

With the conversion to the vidio_ioctl2, tvnorms array is not required anymore.

Also, removed some code from V4L1 time (VIDEO_MODE_foo), specied at the
non-used video_decoder.h.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>