Removing the pgstat_flush_io() call from the walwriter

Hi hackers,

While working on [1], it has been noticed that pgstat_flush_io() is called for
the walwriter. Indeed, it's coming from the pgstat_report_wal() call in
WalWriterMain(). That can not report any I/O stats activity (as the
walwriter is not part of the I/O stats tracking, see pgstat_tracks_io_bktype()).

The behavior is there since 28e626bde00 and I did not find any explicit reason
to do so provided in the linked thread [2].

Calling pgstat_flush_io() from there looks unnecessary, so $SUBJECT, until the
walwriter is part of the I/O stats tracking system.

[1]: https://www.postgresql.org/message-id/Z1rs/j5JMdTbUIJJ%40ip-10-97-1-34.eu-west-3.compute.internal
[2]: https://www.postgresql.org/message-id/flat/20200124195226.lth52iydq2n2uilq%40alap3.anarazel.de

Looking forward to your feedback,

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com