This commit deprecates the following Models\Newsletter methods:
- \MailPoet\Models\Newsletter::filterType()
- \MailPoet\Models\Newsletter::getWelcomeNotificationsForSegments()
- \MailPoet\Models\Newsletter::duplicate()
I'm doing this as part of a ticket to replace the models
NewsletterOption and NewsletterOptionField with Doctrine code. The
methods deprecated here use those models. But since they are not used
anymore in our codebase, I figured it would be easier to deprecate them
instead of replacing the models.
[MAILPOET-4150]
