gnrc_pktbuf: remove gnrc_pktbuf_replace_snip()

The function is unused and unneeded since 69d9ecc and
#5089 calls to provide unittests.

sys/include/net/gnrc/pktbuf.h

@@ -203,13 +203,19 @@ gnrc_pktsnip_t *gnrc_pktbuf_remove_snip(gnrc_pktsnip_t *pkt, gnrc_pktsnip_t *sni
 /**
  * @brief   Replace a snip from a packet and the packet buffer by another snip.
  *
+ * @deprecated  Function is not used by anyone (not even tested, see
+ *              https://github.com/RIOT-OS/RIOT/issues/5089). Will be removed
+ *              after 2020.10 release.
+ *
  * @param[in] pkt   A packet
  * @param[in] old   snip currently in the packet
  * @param[in] add   snip which will replace old
  *
  * @return  The new reference to @p pkt
  */
-gnrc_pktsnip_t *gnrc_pktbuf_replace_snip(gnrc_pktsnip_t *pkt, gnrc_pktsnip_t *old, gnrc_pktsnip_t *add);
+gnrc_pktsnip_t *gnrc_pktbuf_replace_snip(gnrc_pktsnip_t *pkt,
+                                         gnrc_pktsnip_t *old,
+                                         gnrc_pktsnip_t *add);
 
 /**
  * @brief   Reverses snip order of a packet in a write-protected manner.