Remove the sent_notifications_partitioned_reply_key feature flag, use partitioned reply_key format

What does this MR do and why?

The format for the key used in unsubscribe links has changed. The key is also changed in reply-to email addresses that allow replying to an email notification

Removing the feature flag. It has been enabled globally in .com for a week. After this I will introduce a new feature flag to start writing directly on the new partitioned table

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Related to #547622 (closed)