pg_createsubscriber: Use logging.c log file callback

pg_createsubscriber: Use logging.c log file callback

This reverts commit 6b5b7eae3ae, where a new logging API layer was
introduced locally in pg_createsubscriber.  Instead, use the log file
callback introduced in logging.c.  This new approach is simpler,
eliminates code duplication, and doesn't require any caller changes or
NLS updates (which the previous commit missed).

Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEqnbaUthOQARV1dscGvB_EsqC-YfxiM6rWkVDHc%2BG%2Bf4oSUHw%40mail.gmail.com