jetty-12.1.x remove unused code from DefaultSessionIdManager for generation of session id

[janbartel]

The DefaultSessionIdManager uses a workerName to both indicate the identity of the node on which the session is live, and also to prepend to the randomly generated id when creating a new session id.

The workerName is set explicitly via the DefaultSessionIdManager.setWorkerName(String) method, or can also be defaulted to the value of the JETTY_WORKER_INSTANCE environment variable prepended with 'node`.

The code also makes allowance that the workerName can start with a leading $, in which case it is interpreted as the name of a request attribute whose value is the actual worker name (ie identity of the node) . This code seems to have been related to the old mod_jk code, which no longer exists, and it has some undesirable quirks (eg, a workerName with a leading $ will be prepended as-is to the random id instead of the value of the request attribute of that name).

This issue proposes to remove the old code from jetty-12.1 and just use the given or defaulted value of the workerName.

[janbartel]

Fixed by #12719