Remove redundant polling_interval setting
Remove redundant polling_interval setting

This appeared without explanation in [1], but it's the same as the
default value [2] so we don't need to specify it - doing so gives
the impression we made a decision, but that's not clear here.

[1]: https://github.com/alphagov/notifications-api/pull/2142/files#diff-84f1a9419471e289c6b6e2b0209b329e20df6cef81d1f7f0a193ddc2fc6ad69dR153
[2]: https://docs.celeryproject.org/en/stable/getting-started/backends-and-brokers/sqs.html#polling-interval

alphagov/notifications-api,alphagov/notifications-api