We no longer need to assert that the "old approach" and the "ops approach" are equal... because we're removing the "old approach" altogether.

This diff

• removes the last usage of assertMessageStoreMessagesAreEqual from mergeNewMessages
• removes assertMessageStoreMessagesAreEqual
• removes imports that assertMessageStoreMessagesAreEqual depended on
• removes actionType argument from mergeNewMessages(..._) since it was only used by assertMessageStoreMessagesAreEqual from within mergeNewMessages

Depends on D4332

This should be a straightforward refactor given we've had assertMessageStoreMessagesAreEqual in place for some time now.