Clean Up Leftover Multi-Meta

Clean up leftover multi-meta There is a potential bug in the REST API (identified by @MiguelAxcarHM, and being discussed between @TimothyBJacobs and @kadamwhite here in WordPress core Slack) where updates to meta values can fail unexpectedly because of unrelated other meta. Our working theory is that this happens when a meta key is registered (using registermeta or registerpostmeta) with 'single' = true, but that meta key still has multiple rows in the database as you would expect if it was previously used as 'single' = false. For example, This plugin provides a CLI command which can be used to iterate through all posts of a post type, check for registered meta for that post, and identify whether any single-registered meta keys have duplicated rows in the postmeta table. The duplicated meta IDs are then cleaned out. If a meta key has multiple rows for a post with different values, we should leave those alone. Example command: