avdevice/decklink_enc: fix unused variable compiler warnings

Due to refactoring, the ctx/cctx variables are never actually used
in ff_decklink_write_packet(), so just remove them.
Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>

libavdevice/decklink_enc.cpp

@@ -627,8 +627,6 @@ error:
 
 int ff_decklink_write_packet(AVFormatContext *avctx, AVPacket *pkt)
 {
-    struct decklink_cctx *cctx = (struct decklink_cctx *)avctx->priv_data;
-    struct decklink_ctx *ctx = (struct decklink_ctx *)cctx->ctx;
     AVStream *st = avctx->streams[pkt->stream_index];
 
     if      (st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)