Don't use O_SYNC or similar when opening signal file to fsync it.

No need to use get_sync_bit() when we're calling pg_fsync() on the file.
We're not writing to the files, so it doesn't make any difference in
practice, but seems less surprising this way.

Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/b3b71061-4919-e882-4857-27e370ab134a%40iki.fi

src/backend/access/transam/xlog.c

@@ -5484,7 +5484,7 @@ readRecoverySignalFile(void)
 	{
 		int			fd;
 
-		fd = BasicOpenFilePerm(STANDBY_SIGNAL_FILE, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
+		fd = BasicOpenFilePerm(STANDBY_SIGNAL_FILE, O_RDWR | PG_BINARY,
 							   S_IRUSR | S_IWUSR);
 		if (fd >= 0)
 		{
@@ -5497,7 +5497,7 @@ readRecoverySignalFile(void)
 	{
 		int			fd;
 
-		fd = BasicOpenFilePerm(RECOVERY_SIGNAL_FILE, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
+		fd = BasicOpenFilePerm(RECOVERY_SIGNAL_FILE, O_RDWR | PG_BINARY,
 							   S_IRUSR | S_IWUSR);
 		if (fd >= 0)
 		{