sys/net/gnrc/pkt: fix gnrc_pkt_delete()

The previous implementation used creative construct for impedance
mismatching between the core list API (which returns a ptr to the
removed element if found) and the GNRC pkt list API (which returns a
ptr to the new list head) that creates a temporary list head on the
stack.

I'm not entirely sure if the previous implementation is containing
undefined behavior that is used against us with GCC >= 12.x, or if this
is a compiler bug. In either case, not reusing the core list API here
and just having a textbook linked list delete function here is not much
less readable and fixes the issue for our users.

(cherry picked from commit 0d5bde0)

sys/include/net/gnrc/pkt.h

@@ -210,10 +210,26 @@ static inline gnrc_pktsnip_t *gnrc_pkt_prepend(gnrc_pktsnip_t *pkt,
 static inline gnrc_pktsnip_t *gnrc_pkt_delete(gnrc_pktsnip_t *pkt,
                                               gnrc_pktsnip_t *snip)
 {
-    list_node_t list = { (list_node_t *)pkt };
+    /* Removing head is a no-op. The new head is the next in the list. */
+    if (pkt == snip) {
+        return pkt->next;
+    }
+
+    /* Removing nothing is a no-op, the new head is the old one */
+    if (snip == NULL) {
+        return pkt;
+    }
 
-    list_remove(&list, (list_node_t *)snip);
-    return (gnrc_pktsnip_t *)list.next;
+    /* Iterate over the list and remove the given snip from it, if found.
+     * The new head is the old head. */
+    for (gnrc_pktsnip_t *i = pkt; i != NULL; i = i->next) {
+        if (i->next == snip) {
+            i->next = snip->next;
+            return pkt;
+        }
+    }
+
+    return pkt;
 }
 
 /**