Review current events for removal or deprecation

[mariajgrimaldi]

Description

There are a few events that are not being used in any service that should be reviewed to decide whether to drop support:

• COURSE_CERTIFICATE_CONFIG_CHANGED
• COURSE_CERTIFICATE_CONFIG_DELETED
• SUBSIDY_REDEEMED
• SUBSIDY_REDEMPTION_REVERSED

And others where we should start a deprecation process because the event was replaced by a new one:

• CONTENT_OBJECT_TAGS_CHANGED

Definition of Done

• The team that implemented the event is aware of the event's status
• The events were implemented in a service, removed or deprecated.

[justinhynes]

For what it's worth, I've recently proposed a possible use case for the COURSE_CERTIFICATE_CONFIG_CHANGED event in #477.