250-300MB is really a lot and I’m not sure if that is a normal situation. I’d suggest you to check how was that site maintained and managed. Maybe the updates were not made correctly (for example plugins were not updated and only overwritten using FTP).
And back to your question… The easiest way to clean these files would be to:
- Download original, current version from Plugin Repo: https://wordpress.org/plugins/wordpress-seo/
- Login to your FTP server and go to wp-content/plugins/
- Rename wordpress-seo folder
- Upload original folder from downloaded package
- Remove old (renamed) folder.