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>