Remove redundant rtinit() calls from tuntap.

Removed code iterates over if_addrhead and tries to remove
routes for each ifa.
This is exactly the thing that if_purgeaddrs() do, and
if_purgeaddr() is already called in the end.

Reviewed by: glebius
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D28106