Use IP_FW_NAT44_DESTROY opcode for IP_FW3 socket option to destroy

Use IP_FW_NAT44_DESTROY opcode for IP_FW3 socket option to destroy
NAT instance.

The NAT44 group of opcodes for IP_FW3 socket option is modern way
to control NAT instances and this method can be used in future to
switch from numeric to named NAT instances, like was done for ipfw
tables.
The IP_FW_NAT_DEL opcode is the last remnant of old ipfw_ctl control
plane that doesn't support versioned operations. This interface will
be retired soon.

Reviewed by:	melifaro
MFC after:	10 days
Sponsored by:	Yandex LLC