Do not turn on the vcr_hack based on the btv->field value.

This was a change in the bttv vb2 conversion that caused
green lines at the bottom of the picture in tvtime.

It was originally added to the vb2 conversion based on
faulty information that without this there would be glitches
in the video. However, later tests suggest that this is a
problem in the utilities used to test this since tvtime
behaves fine.

This patch reverts the bttv driver to the original pre-vb2
behavior w.r.t. vcr_hack.

Fixes: b7ec3212 ("media: bttv: convert to vb2")
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>