media: uapi: h264: Drop SLICE_PARAMS 'size' field

The SLICE_PARAMS control is intended for slice-based
devices. In this mode, the OUTPUT buffer contains
a single slice, and so the buffer's plane payload size
can be used to query the slice size.

To reduce the API surface drop the size from the
SLICE_PARAMS control.

A follow-up change will remove other members in SLICE_PARAMS
so we don't need to add padding fields here.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>