[PATCH] Fix error message in RemoveWalSummaryIfOlderThan to indicate file removal failure

Hi all,

I noticed a small issue. In `RemoveWalSummaryIfOlderThan`,an `unlink()` failure incorrectly logs "could not stat file", likely a copy-paste error.
I updated it to "could not remove file". No functional changes.

```
diff --git a/src/backend/backup/walsummary.c b/src/backend/backup/walsummary.c
index 21164faac7e..4ee510092f9 100644
--- a/src/backend/backup/walsummary.c
+++ b/src/backend/backup/walsummary.c
@@ -251,7 +251,7 @@ RemoveWalSummaryIfOlderThan(WalSummaryFile *ws, time_t cutoff_time)
 	if (unlink(path) != 0)
 		ereport(ERROR,
 				(errcode_for_file_access(),
-				 errmsg("could not stat file \"%s\": %m", path)));
+				 errmsg("could not remove file \"%s\": %m", path)));
 	ereport(DEBUG2,
 			(errmsg_internal("removing file \"%s\"", path)));
 }
```

--
regards,
Man Zeng