media: rkvdec: Remove TODO file

2 items are present in the TODO file:
 - HEVC support
 - Evaluate adding helper for rkvdec_request_validate

Missing HEVC support is not a reason for a driver to be in staging,
support for different features of the hardware can be added in drivers
in the main tree.

The rkvdec_request_validate function was simplified in
commit 54676d5f ("media: rkvdec: Do not require all controls to be present in every request")
by not setting controls that have not changed.
As it now basically just calls vb2_request_validate(), there is no need
for a helper.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>