media: Documentation: vivid.rst: Remove documentation for Capture Overlay

Modifying documentation to remove 'Capture Overlay section' as
destructive capture overlay support was removed.

See commit ccaa9d50 ("media: vivid: drop overlay support")

Signed-off-by: Dorcas Anono Litunya <anonolitunya@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>