Fix error message in RemoveWalSummaryIfOlderThan()

A failing unlink() was reporting an incorrect error message, referring
to stat().

Author: Man Zeng <zengman@halodbtech.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://postgr.es/m/tencent_3BBE865C5F49D452360FF190@qq.com
Backpath-through: 17

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)));
 }