V4L/DVB (11298): cx25840: remove legacy code for old-style i2c API

All drivers that use cx25840 are now converted to v4l2_subdev, so I can remove
the support for the old-style i2c API.

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