aio: Remove obsolete IO worker ID references.

aio: Remove obsolete IO worker ID references.

In an ancient ancestor of this code, the postmaster assigned IDs to IO
workers.  Now it tracks them in an unordered array and doesn't know
their IDs, so it might be confusing to readers that it still referred to
their indexes as IDs.

No change in behavior, just variable name and error message cleanup.

Back-patch to 18.

Discussion: https://postgr.es/m/CA%2BhUKG%2BwbaZZ9Nwc_bTopm4f-7vDmCwLk80uKDHj9mq%2BUp0E%2Bg%40mail.gmail.com