Review the usefulness of EntityDatasourceInterface

While working on #2007692: Changes in related entities don't lead to proper re-indexing, it occurred to me that the \Drupal\search_api\Plugin\search_api\datasource\EntityDatasourceInterface interface seems pretty useless. It neither seems to be used anywhere (except for implementing it, and for some reason checking that the entity:* datasources do actually implement it), nor do I know of any other module implementing it with a datasource plugin.

So, I’d probably just deprecate the whole thing?
Or, at the very least, the static method, which is particularly useless on an interface.