avcodec/internal: remove unused out_frame field from DecodeSimpleContext

It was introduced in 061a0c14 but apparently never used
Signed-off-by: NJames Almer <jamrial@gmail.com>

libavcodec/internal.h

@@ -110,7 +110,6 @@
 
 typedef struct DecodeSimpleContext {
     AVPacket *in_pkt;
-    AVFrame  *out_frame;
 } DecodeSimpleContext;
 
 typedef struct EncodeSimpleContext {