Remove pgstat_flush_wal()

All the processes that generate WAL should call pgstat_report_wal() to
report all their statistics related to WAL, and this is already what
happens in the tree.  Keeping pgstat_report_wal() is confusing while the
other routine is encouraged.

This routine is not required since fc415edf8c, where it was lastly
used in pgstat_report_stat() before an equivalent callback existed.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z71oPkJJICrRB5Ws@paquier.xyz

src/backend/utils/activity/pgstat_wal.c

@@ -51,7 +51,7 @@ pgstat_report_wal(bool force)
 	nowait = !force;
 
 	/* flush wal stats */
-	pgstat_flush_wal(nowait);
+	(void) pgstat_wal_flush_cb(nowait);
 
 	/* flush IO stats */
 	pgstat_flush_io(nowait);
@@ -69,15 +69,6 @@ pgstat_fetch_stat_wal(void)
 	return &pgStatLocal.snapshot.wal;
 }
 
-/*
- * Simple wrapper of pgstat_wal_flush_cb()
- */
-void
-pgstat_flush_wal(bool nowait)
-{
-	(void) pgstat_wal_flush_cb(nowait);
-}
-
 /*
  * Calculate how much WAL usage counters have increased by subtracting the
  * previous counters from the current ones.

src/include/utils/pgstat_internal.h

@@ -748,8 +748,6 @@ extern void pgstat_slru_snapshot_cb(void);
  * Functions in pgstat_wal.c
  */
 
-extern void pgstat_flush_wal(bool nowait);
-
 extern void pgstat_wal_init_backend_cb(void);
 extern bool pgstat_wal_have_pending_cb(void);
 extern bool pgstat_wal_flush_cb(bool nowait);