pgsql: Remove buffers_backend and buffers_backend_fsync from pg_stat_ch

Remove buffers_backend and buffers_backend_fsync from pg_stat_checkpointer

Two attributes related to checkpointer statistics are removed in this
commit:
- buffers_backend, that counts the number of buffers written directly by
a backend.
- buffers_backend_fsync, that counts the number of times a backend had
to do fsync() by its own.

These are actually not checkpointer properties but backend properties.
Also, pg_stat_io provides a more accurate and equivalent report of these
numbers, by tracking all the I/O stats related to backends, including
writes and fsyncs, so storing them in pg_stat_checkpointer was
redundant.

Thanks also to Robert Haas and Amit Kapila for their input.

Bump catalog version.

Author: Bharath Rupireddy
Reviewed-by: Bertrand Drouvot, Andres Freund
Discussion: https://postgr.es/m/20230210004604.mcszbscsqs3bc5nx@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/74604a37f2f9eb6e626a4ff3cedd02aef5a2ad59

Modified Files
--------------
doc/src/sgml/monitoring.sgml | 20 ---------------
src/backend/catalog/system_views.sql | 2 --
src/backend/postmaster/checkpointer.c | 32 ++----------------------
src/backend/utils/activity/pgstat_checkpointer.c | 4 ---
src/backend/utils/adt/pgstatfuncs.c | 12 ---------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 9 -------
src/include/pgstat.h | 2 --
src/test/regress/expected/rules.out | 2 --
9 files changed, 3 insertions(+), 82 deletions(-)