[media] dev-capture.rst/dev-output.rst: video standards ioctls are optional

The documentation for video capture and output devices claims that the video standard
ioctls are required. This is not the case, they are only required for PAL/NTSC/SECAM
type inputs and outputs. Sensors do not implement this at all and e.g. HDMI inputs
implement the DV Timings ioctls.

Just drop the mention of 'video standard' ioctls.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>