[media] media/video/et61x251: Remove this deprecated driver

The et61x251 has been deprecated for a couple of releases now, as all
devices it supports are also supported by gspca_etoms, and it has not
seen any maintenance in years. So now it is time to remove it.

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