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

This issue has been migrated from [#11829](https://github.com/matrix-org/synapse/issues/11829).

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I ran into this while implementing #11215.

Before https://github.com/matrix-org/synapse/pull/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...