Delete Rake Task to Backfill contract_overages_allowed__c

Delete the rake task that was used to backfill zuora_subscriptions.contract_overages_allowed__c.

The backfill was run on Staging and Production, so we don't need it anymore.

The rake task is here: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/31f0641b3a054b33f822e1bfb5805fb2284bbde9/lib/tasks/data_maintenance/backfill_zuora_local_subscription_contract_overages_allowed.rake