[media] Remove documentation chunk of non-existent V4L2_CID_AUTO_FOCUS_AREA

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: NSylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/compat.xml

@@ -2614,10 +2614,6 @@ ioctls.</para>
 	  <para>Sub-device selection API: &VIDIOC-SUBDEV-G-SELECTION;
 	  and &VIDIOC-SUBDEV-S-SELECTION; ioctls.</para>
         </listitem>
-        <listitem>
-	  <para><link linkend="v4l2-auto-focus-area"><constant>
-	  V4L2_CID_AUTO_FOCUS_AREA</constant></link> control.</para>
-        </listitem>
         <listitem>
 	  <para>Support for frequency band enumeration: &VIDIOC-ENUM-FREQ-BANDS; ioctl.</para>
         </listitem>