In the past we checked remote binary mirrors for existence of a spec
before attempting to download it. That changed to only checking local
copies of index.jsons (if available) to prioritize certain mirrors where
we expect to find a tarball. That was faster for CI since fetching
index.json and loading it just to order mirrors takes more time than
just attempting to fetch tarballs -- and also if we have a direct hit
there's no point to look at other mirrors.
Long story short: the info message only makes sense in the old version
of Spack, so it's better to remove it.
3bc943ae51