Outdated indexes are not cleaned up

[roborourke]

In some cases, specifically after upgrading from a version of ElasticPress prior to the introduction of indexables it can leave old indexes behind that it no longer recognises and that are not removed by reindexing. They are benign until we need to upgrade ES as currently the old indexes have to be manually removed.

It may be easier to do the manual removal if there aren't too many but might be worth adding a cleanup routine to the wp altis migrate command.

It should remove any indexes that are:

• not the global aliases
• begin with ep-
• are not in the list of indexes given by wp elasticpress get-indexes