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

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202310261
+#define CATALOG_VERSION_NO	202310271
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5738,15 +5738,6 @@
   proname => 'pg_stat_get_checkpoint_sync_time', provolatile => 's',
   proparallel => 'r', prorettype => 'float8', proargtypes => '',
   prosrc => 'pg_stat_get_checkpoint_sync_time' },
-{ oid => '2775', descr => 'statistics: number of buffers written by backends',
-  proname => 'pg_stat_get_buf_written_backend', provolatile => 's',
-  proparallel => 'r', prorettype => 'int8', proargtypes => '',
-  prosrc => 'pg_stat_get_buf_written_backend' },
-{ oid => '3063',
-  descr => 'statistics: number of backend buffer writes that did their own fsync',
-  proname => 'pg_stat_get_buf_fsync_backend', provolatile => 's',
-  proparallel => 'r', prorettype => 'int8', proargtypes => '',
-  prosrc => 'pg_stat_get_buf_fsync_backend' },
 { oid => '2859', descr => 'statistics: number of buffer allocations',
   proname => 'pg_stat_get_buf_alloc', provolatile => 's', proparallel => 'r',
   prorettype => 'int8', proargtypes => '', prosrc => 'pg_stat_get_buf_alloc' },

src/include/pgstat.h

@@ -265,8 +265,6 @@ typedef struct PgStat_CheckpointerStats
 	PgStat_Counter checkpoint_write_time;	/* times in milliseconds */
 	PgStat_Counter checkpoint_sync_time;
 	PgStat_Counter buf_written_checkpoints;
-	PgStat_Counter buf_written_backend;
-	PgStat_Counter buf_fsync_backend;
 } PgStat_CheckpointerStats;