Remove backwards-compatibility code for access tokens without an associated device

[matrixbot]

This issue has been migrated from #11829.

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I ran into this while implementing #11215.

Before matrix-org/synapse#949, it was possible to have access tokens that were not associated with a device. This has now been deprecated for 5 years.

This possibility has now crept across the codebase, as we've marked device_id as Optional in many places, including:

https://github.com/matrix-org/synapse/blob/d4e6f358b64b340a670e4f7496a530c11a715920/synapse/handlers/sync.py#L87-L93

This makes little sense, as you're expected to have a device if you're calling /sync. Much of the /sync handling code assumes you have a device, and would logically fail if the user didn't:

https://github.com/matrix-org/synapse/blob/3e0536cd2afb5a640619bd872fc27b068ec3eb9b/synapse/storage/databases/main/deviceinbox.py#L199-L201

Passing device_id=None to this function always results in zero to-device messages being returned.

Is there anything holding us back (old access tokens?) from marking device IDs as str, not Optional[str]?

Edit: There are currently 64 entries in the access_tokens table on matrix.org that do not have an associated device ID. They are all either abuse, or go neb...