Attempting to resync the device list for non existing user after federation turned off.

This issue has been migrated from [#10218](https://github.com/matrix-org/synapse/issues/10218).

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### Description

After disabling federation for my homeserver, I get logs related to device lists resync, for users that are no more in any rooms.
Using nginx as reverse proxy, I have disabled federation by removing 8448 port from nginx conf file, set `federation_domain_whitelist =[]`, and denied 8448 in ufw firewall.
A log entry example would be:
```
2021-06-18 12:26:27,235 - synapse.handlers.device - 844 - DEBUG - _maybe_retry_device_resync-106 - Attempting to resync the device list for @philipp:netzgemeinde.eu
2021-06-18 12:26:27,235 - synapse.federation.federation_client - 175 - DEBUG - _maybe_retry_device_resync-106 - Invoked 'query_user_devices' with args: self=