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.