Removed function prototype that had no corresponding implementation (pbuf_ref_chain)

src/include/lwip/pbuf.h

@@ -102,7 +102,6 @@ struct pbuf *pbuf_alloc(pbuf_layer l, u16_t size, pbuf_type type);
 void pbuf_realloc(struct pbuf *p, u16_t size); 
 u8_t pbuf_header(struct pbuf *p, s16_t header_size);
 void pbuf_ref(struct pbuf *p);
-void pbuf_ref_chain(struct pbuf *p);
 u8_t pbuf_free(struct pbuf *p);
 u8_t pbuf_clen(struct pbuf *p);  
 void pbuf_cat(struct pbuf *head, struct pbuf *tail);