pgsql: 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

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b4c19da93a08f767f88de9696af0de3b1715d40e

Modified Files
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src/backend/postmaster/postmaster.c | 28 ++++++++++++++--------------
1 file changed, 14 insertions(+), 14 deletions(-)