That method will be used by plugin Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8496>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4288>
It seems that `last_output_poc` in `h264decoder` class is a left over of commit 5527cc4a2e7. This patch removes it but keeps the log message by fetching the `h264picture`'s `last_output_pic`. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4288>