media: hantro: Remove dedicated control documentation

The dedicated control required by the HEVC support
was removed, and the driver now calculates the value
internally. Remove the ad-hoc documentation as well.

[hverkuil: remove hantro from Documentation/userspace-api/media/drivers/index.rst]

Fixes: 3360755e

 ("media: hantro: Stop using Hantro dedicated control")
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>