Remove duplicate initialization in initialize_brin_buildstate().

Commit dae761a added initialization of some BrinBuildState fields
in initialize_brin_buildstate(). Later, commit b437571 inadvertently
added the same initialization again.

This commit removes that redundant initialization. No behavioral
change is intended.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Discussion: https://postgr.es/m/CAEoWx2nmrca6-9SNChDvRYD6+r==fs9qg5J93kahS7vpoq8QVg@mail.gmail.com

src/backend/access/brin/brin.c

@@ -1689,9 +1689,6 @@ initialize_brin_buildstate(Relation idxRel, BrinRevmap *revmap,
 	state->bs_leader = NULL;
 	state->bs_worker_id = 0;
 	state->bs_sortstate = NULL;
-	state->bs_context = CurrentMemoryContext;
-	state->bs_emptyTuple = NULL;
-	state->bs_emptyTupleLen = 0;
 
 	/* Remember the memory context to use for an empty tuple, if needed. */
 	state->bs_context = CurrentMemoryContext;