Remove outdated constraint on remote_media_cache_thumbnails (#9725)

The `remote_media_cache_thumbnails_media_origin_media_id_thumbna_key`
constraint is superceded by
`remote_media_repository_thumbn_media_origin_id_width_height_met` (which adds
`thumbnail_method` to the unique key).

PR #7124 made an attempt to remove the old constraint, but got the name wrong,
so it didn't work. Here we update the bg update and rerun it.

Fixes #8649.