Some Mastodon versions (v1.1 and v1.2) had a duplicate index in `db/schema.rb`
without any migration script creating it. #2224 (included in v1.3) removed the
duplicate index from the file but did not provide a migration script to remove
it.
This means that any instance that was installed from v1.1 or v1.2's source code
has a duplicate index and a corresponding warning in PgHero. Instances set up
using an earlier or later Mastodon version do not have this issue.
This PR removes the duplicate index if it is present.
Nick Schonning