brin: Remove duplicate initialization in initialize_brin_buildstate()

Hi Hacker,

Just noticed a redundant assignment in initialize_brin_buildstate():

```
static BrinBuildState *
initialize_brin_buildstate(Relation idxRel, BrinRevmap *revmap,
BlockNumber pagesPerRange, BlockNumber tablePages)
{
BrinBuildState *state;
BlockNumber lastRange = 0;

state = palloc_object(BrinBuildState);

<…omit some lines…>

// Later, b437571 added the same again. This patch deletes these 3 lines.
state->bs_context = CurrentMemoryContext;
state->bs_emptyTuple = NULL;
state->bs_emptyTupleLen = 0;

// Added by dae761a first
/* Remember the memory context to use for an empty tuple, if needed. */
state->bs_context = CurrentMemoryContext;
state->bs_emptyTuple = NULL;
state->bs_emptyTupleLen = 0;
```

So, filing a trivial patch to eliminate the redundancy.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/