Move read-only-archive hint from "notmuch setup" to "notmuch new"

Move read-only-archive hint from "notmuch setup" to "notmuch new"

The "notmuch setup" output was getting overwhelmingly verbose.

Also, some people might not have a lot of mail, so might never need
this optimization. It's much better to move the hint to the time
when the user could actually benefit from it, (it's easy to detect
that "notmuch new" took more than 1 second, and we know if there
are any read-only directories there or not).