* Remove sustaining member newsletter, and its settings
This removes the concept of sustaining memberships from the system,
and logic related to or dependent on it.
This does not remove the monthly_dues column from the users table (todo).
* Remove unused newsletter setting
Since nothing accesses the mailchimp_sustaining_members_id setting,
it's safe to remove.
* Use destroy rather than delete to ensure settings cache is cleared
We have a callback in Settings::Base to clear the cache after commit,
I assume it's useful to trigger that. This requires destroy, not
delete, to be called.
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