Ah, fall harvest -- when farmers gather their crops and network execs cut their crap. CBS chucked its first rotten potato last week: David E. Kelley's ''Brotherhood of Poland, New Hampshire,'' which averaged 6.9 million viewers (specials will fill the Wednesday slot). And the barn door couldn't close fast enough on ''Luis,'' which averaged 3.5 million (to fill the Friday void, Fox will temporarily double-pump ''Wanda at Large''). Oh, and UPN's ''The Mullets'' is on ''hiatus'' (yeah, right). One net topper admits it's not easy to send new shows out to pasture, ''but it's more embarrassing to get bad ratings every week.''