ng_iface(4): Remove a redundant assignment of if_type

That is unnecessary since change [1], as if_alloc(IFT_PROPVIRTUAL) has
set if_type already.

[1] fc74a9f93a5f Stop embedding struct ifnet at the top of driver softcs

No functional change intended.

MFC after: 1 week

(cherry picked from commit c43d6dadf7e34a94f4d119ff6d7113ad9a9b4d38)