Remove duplicate code in brin_memtuple_initialize

Commit 8bf7496 moved some of the code from brin_new_memtuple to
brin_memtuple_initialize, but this resulted in some of the code being
duplicate. Fix by removing the duplicate lines and backpatch to 10.

Author: Tomas Vondra
Backpatch-through: 10
Discussion: https://postgr.es/m/5eb50c97-9a8e-b691-8c40-1b2a55611c4c%40enterprisedb.com

Author: tvondra

src/backend/access/brin/brin_tuple.c

@@ -492,9 +492,6 @@ brin_memtuple_initialize(BrinMemTuple *dtuple, BrinDesc *brdesc)
 				 sizeof(BrinValues) * brdesc->bd_tupdesc->natts);
 	for (i = 0; i < brdesc->bd_tupdesc->natts; i++)
 	{
-		dtuple->bt_columns[i].bv_allnulls = true;
-		dtuple->bt_columns[i].bv_hasnulls = false;
-
 		dtuple->bt_columns[i].bv_attno = i + 1;
 		dtuple->bt_columns[i].bv_allnulls = true;
 		dtuple->bt_columns[i].bv_hasnulls = false;