Removing if_type switches in if_vlan.c

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

There are several if_type switches in if_vlan.c,
which were introduced to support other hardware
types such as FDDI many years ago. However, no
implement hadn't come since then.

I think there is no reason to keep them and
I want to get rid of them to improve code
readability.

Any objections?

  ozaki-r