[media] pwc: Remove dead snapshot code

The in kernel version of the pwc driver has never supported snapshot
mode, and now that we no longer support the pixfmt.priv abuse there also
no longer is a way for userspace to request it, rendering all the code in
question dead (never called), so remove it.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>