Do not pass PDO::FETCH_NUM (integer 3) as limit parameter to OC_DB::prepare().

It's pretty clear that passing PDO::FETCH_NUM was never intended. Passing a limit is pointless as aggregation without group by returns only one row.

Discovered in #5000
Regression from 0e2b957dacb86c510c59721f63879f1c9d93d5d4

@DeepDiver1975 @icewind1991 @karlitschek @ringmaster @kabum