Remove superfluous memset from pgstat_recv_resetslrucounter

The extra memset meant pg_stat_reset_slru() always reset all the entries
even when reset of a single entry was requested, but the timestamp was
left uninitialized.

Reported-by: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYFe16pjZxQYaTn53mspyM7dgMPYL3DJLjjPw69GMCC2Ow%40mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -6233,8 +6233,6 @@ pgstat_recv_resetslrucounter(PgStat_MsgResetslrucounter *msg, int len)
 	int			i;
 	TimestampTz	ts = GetCurrentTimestamp();
 
-	memset(&slruStats, 0, sizeof(slruStats));
-
 	for (i = 0; i < SLRU_NUM_ELEMENTS; i++)
 	{
 		/* reset entry with the given index, or all entries (index is -1) */