“Got a certificate for $DIRAUTH, but we already have it.” should not be a warning

A bridge operator reported the following message on tor-talk:

[warn] Got a certificate for dizum, but we already have it. Maybe they haven't updated it. Waiting for a while.

It's not something that normal users should care about, so we shouldn't show it to them (i.e. we should downgrade it to info level). (Perhaps it should stay at notice level on dirauths, though.)