Previous versions of assets not deleted when asset filename changes

Problem/Motivation

When a media type is set to Download and sync assets, and the asset filename from the DAM has changed since the asset was last synced to Drupal, when the sync runs the new file is downloaded and associated with the media item, but the old file remains on the filesystem.

Steps to reproduce

1. Configure a media type to Download and sync assets
2. Sync an asset of that media type to Drupal, noting the file name
3. Upload a new version of the file to the DAM, using a different file name
4. Re-sync the asset
5. In the Drupal file listing (/admin/content/files), you will see both the old file name and the new file name present as Drupal files

This case should probably be handled in \Drupal\acquia_dam\AssetFileEntityHelper::downloadFile().