☂ Remove various _filename records.config variables.

This turned out to be a little more complicated than expected, because these records.config are actually used as part of the mechanisms for reloading configuration files. But we should get this done for ATS v10.

- [x] proxy.config.cache.storage_filename
- [ ] proxy.config.cache.control.filename
- [ ] proxy.config.cache.ip_allow.filename
- [ ] proxy.config.cache.hosting_filename
- [ ] proxy.config.cache.volume_filename
- [ ] proxy.config.dns.splitdns.filename
- [ ] proxy.config.hostdb.filename
- [ ] proxy.config.log.config.filename
- [ ] proxy.config.url_remap.filename
- [ ] proxy.config.ssl.server.multicert.filename
- [ ] proxy.config.ssl.servername.filename

The argument is that
1. This doesn't fly well with the move to a unified YAML namespace
2. This functionality is superseded with the traffic layout / runroot features.