net/pkt: Remove useless _reserved attribute

This attribute was used when net_pkt where queued into a fifo. However,
since the cove to k_work queues (due to the introduction of TC), such
attribute has been totally useless.

Removing it gains 4 bytes in net_pkt structure.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

include/net/net_pkt.h

@@ -58,9 +58,6 @@ struct net_pkt_cursor {
  * net_pkt_clone() function.
  */
 struct net_pkt {
-	/** FIFO uses first 4 bytes itself, reserve space */
-	int _reserved;
-
 	/** Internal variable that is used when packet is sent */
 	struct k_work work;