bridge: Remove a redundant assignment of if_type

ether_ifattach() no longer sets if_type to IFT_ETHER and keeps it as is
since the change [1].

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

No functional change intended.

MFC after: 1 week

(cherry picked from commit a07604e6264b88222941fa61c6f989bad5490765)