V4L/DVB (11366): v4l: remove obsolete header and source

v4l2-subdev.c and v4l2-i2c-drv-legacy.h were used to support the old i2c
API. All v4l drivers are now converted to v4l2_subdev, so these two files
can be removed.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>