picture_chain: remove unused vlc_picture_chain_AppendChain

It was only temporary until we can properly drain pictures from filters. Now a
filter must never return chained pictures. It must implement a drain instead.

include/vlc_picture.h

@@ -266,14 +266,6 @@ static inline void vlc_picture_chain_Append(vlc_picture_chain_t *chain,
     chain->tail = pic;
 }
 
-/**
- * Append a picture chain to a picture chain.
- */
-static inline void vlc_picture_chain_AppendChain(picture_t *chain, picture_t *tail)
-{
-    chain->p_next = tail;
-}
-
 /**
  * Copy the picture chain in another picture chain and clear the original
  * picture chain.