Remove documentation chunk of non-existent V4L2_CID_AUTO_FOCUS_AREA
control. It fixes following build error:

Error: no ID for constraint linkend: v4l2-auto-focus-area.
Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>