Prevent redundant API call when using pg:info

Because of the way [database add-ons are collected][1], you end up with a duplicate
for the `DATABASE_URL` database. The client then goes on and asks API for information on all these databases, including asking about the duplicate database. If you have 2 databases, it makes 3 API calls.

This update rejects that entry, and only hits the API once for each database add-on.

Before:

```bash
$ bin/heroku pg:info -a puma-example
RestClient.get "https://***"
# => 200 OK | text/html 556 bytes
RestClient.get "https://***"
# => 200 OK | text/html 571 bytes
RestClient.get "https://***"
# => 200 OK | text/html 571 bytes
=== HEROKU_POSTGRESQL_IVORY_URL (DATABASE_URL)
Plan:        Crane
[...]

=== HEROKU_POSTGRESQL_ONYX_URL
Plan:        Dev
[...]
```

After:

```bash
$ bin/heroku pg:info -a puma-example
RestClient.get "https://***"
# => 200 OK | text/html 556 bytes
RestClient.get "https://***"
# => 200 OK | text/html 571 bytes
=== HEROKU_POSTGRESQL_IVORY_URL (DATABASE_URL)
Plan:        Crane
[...]

=== HEROKU_POSTGRESQL_ONYX_URL
Plan:        Dev
[…]
```

[1]: https://github.com/heroku/heroku/blob/master/lib/heroku/helpers/heroku_postgresql.rb#L91-L102