Remove some more useless assignments.

Found with clang's scan-build tool.  It also whines about a lot of
other dead stores that we should *not* change IMO, either as a matter
of style or future-proofing.  But these places seem like clear
oversights.

Discussion: https://postgr.es/m/CAEudQAo1+AcGppxDSg8k+zF4+Kv+eJyqzEDdbpDg58-=MQcerQ@mail.gmail.com

src/backend/access/brin/brin_tuple.c

@@ -243,7 +243,6 @@ brin_form_tuple(BrinDesc *brdesc, BlockNumber blkno, BrinMemTuple *tuple,
 
 			*bitP |= bitmask;
 		}
-		bitP = ((bits8 *) (rettuple + SizeOfBrinTuple)) - 1;
 	}
 
 	if (tuple->bt_placeholder)

src/backend/access/gin/ginbtree.c

@@ -241,7 +241,6 @@ ginFindParents(GinBtree btree, GinBtreeStack *stack)
 
 	blkno = root->blkno;
 	buffer = root->buffer;
-	offset = InvalidOffsetNumber;
 
 	ptr = (GinBtreeStack *) palloc(sizeof(GinBtreeStack));