Remove redundant initialization of BrinMemTuple.

brin_new_memtuple already did this, so there's no need
for initialize_brin_buildstate to do it again.

Richard Guo, reviewed by Bharath Rupireddy

Discussion: https://postgr.es/m/CAMbWs4-kYYpKNOdiWtsCZ3jbkFFj4nhOVH22JH7dsrMYX=aGjg@mail.gmail.com

src/backend/access/brin/brin.c

@@ -1262,8 +1262,6 @@ initialize_brin_buildstate(Relation idxRel, BrinRevmap *revmap,
 	state->bs_bdesc = brin_build_desc(idxRel);
 	state->bs_dtuple = brin_new_memtuple(state->bs_bdesc);
 
-	brin_memtuple_initialize(state->bs_dtuple, state->bs_bdesc);
-
 	return state;
 }