avcodec/adpcmenc: remove forward declaration

Reviewed-by: Limin Wang <lance.lmwang@gmail.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/adpcmenc.c

@@ -58,8 +58,6 @@ typedef struct ADPCMEncodeContext {
 
 #define FREEZE_INTERVAL 128
 
-static av_cold int adpcm_encode_close(AVCodecContext *avctx);
-
 static av_cold int adpcm_encode_init(AVCodecContext *avctx)
 {
     ADPCMEncodeContext *s = avctx->priv_data;