[identity] Fix log_out_user DDB expression error

Summary:
Noticed that log_out_user fails when deleting device from legacy DDB table. At least on Localstack.
The error thrown from update_item() call is as following:

ServiceError(ServiceError { source: UpdateItemError { kind: Unhandled(Unhandled { source: Error { code: Some("ValidationException"), message: Some("Invalid UpdateExpression: Internal server error"), ...)
`

Reading AWS docs, I noticed they either hardcode map keys or use expressio attribute names for map keys, not values. This solved the issue.
docs, example SO answer

Test Plan:
Error log diappears in D10422 Commtest CI job after I rebased on this diff.
That diff is the first one that uses log_out_user function.

Reviewers: varun, michal, kamil

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10423