Don't create "contact" terms in the database.

We never did export any interface to get at these, and when I went to
use these, I found them inadequate, (because I wanted to distinguish
address found in from: from those found in To:). Meanwhile, it was
easy enough to extract addresses with a search like:

	notmuch show tag:sent | grep ^To:

so the storage of contact terms was just wasting space. Stop that.