net: lwm2m: Handle RESET replies for separate response

Separate CON responses do no have a reply callback registered (as no
response is expected), however are still registered for retransmission
on the pending list. Therefore, we not only need to check for empty
ACKs for such case but for RESET packets as well. However, this cannot
be done before "reply" processing, as some of the reply handlers
(notifications namely) check for RESET replies.

Therefore, add a final check for the RESET response after the existing
"reply" logic. In such case, just remove the pending response packet
from the retransmission queue, and release resources.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>