Post meta not removed when emails are deleted

[donnapep]

Steps to Reproduce

1. Run the following SQL query. In an ideal world, there would be 19 rows returned - select * from wp_postmeta where meta_key like '%sensei_email%'
2. In Sensei LMS > Tools, click Recreate Emails.
3. Re-run the query.

What I Expected

The previous post meta to be deleted.

What Happened Instead

Many more rows are returned and the previous post meta remains.

We should also check if we're deleting email data when the plugin is uninstalled.