media: video_function_calls.rst: drop obsolete video-set-attributes reference

This fixes this warning:

Documentation/media/uapi/dvb/video_function_calls.rst:9: WARNING: toctree contains
reference to nonexisting document 'uapi/dvb/video-set-attributes'

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>