media: don't set AVCodecContext->refcounted_frames

This attribute was useful with the old decoding API. Refcounted frames
are always used with the avcodec_send_packet/avcodec_receive_frame API.

Change-Id: I0904ed8731c329e0f8f3d59d1872bff6a26543be
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>