tests: net: tcp2: Avoid trying to free net_pkt twice

For success cases, the net_pkt is handled by the IP stack
so no need to free the net_pkt. The double free is not causing
any issues as the net_pkt code will just ignore the request but
we should avoid doing it because of error prints that are printed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>