Media files in `url_cache/` and `url_cache_thumbnails/` should not be stored in storage providers

### Description

Spotted by @richvdh:

Currently media files in `url_cache/` and `url_cache_thumbnails/` get replicated to storage providers.

These files are supposed to be transient, and so should not be stored in any storage providers (eg. S3)

### Version information

- **Homeserver**: matrix.org