Assign TupleTableSlot->tts_tableOid duplicated in tale AM.

Hi hackers,

Recently, we discover that the field of tts_tableOid of TupleTableSlot is
assigned duplicated in table AM's interface which is not necessary. For
example, in table_scan_getnextslot,

```
static inline bool
table_scan_getnextslot(TableScanDesc sscan, ScanDirection direction,
TupleTableSlot *slot)
{
slot->tts_tableOid = RelationGetRelid(sscan->rs_rd);

/*
* We don't expect direct calls to table_scan_getnextslot with valid
* CheckXidAlive for catalog or regular tables. See detailed
comments in
* xact.c where these variables are declared.
*/
if (unlikely(TransactionIdIsValid(CheckXidAlive) && !bsysscan))
elog(ERROR, "unexpected table_scan_getnextslot call during
logical decoding");

return sscan->rs_rd->rd_tableam->scan_getnextslot(sscan, direction,
slot);
}
```

we can see that it assigns tts_tableOid, while calling
sscan->rs_rd->rd_tableam->scan_getnextslot which implemented by
heap_getnextslot also assigns tts_tableOid in the call of
ExecStoreBufferHeapTuple.

```
TupleTableSlot *
ExecStoreBufferHeapTuple(HeapTuple tuple,
TupleTableSlot *slot,
Buffer buffer)
{
/*
* sanity checks
*/
Assert(tuple != NULL);
Assert(slot != NULL);
Assert(slot->tts_tupleDescriptor != NULL);
Assert(BufferIsValid(buffer));

if (unlikely(!TTS_IS_BUFFERTUPLE(slot)))
elog(ERROR, "trying to store an on-disk heap tuple into
wrong type of slot");
tts_buffer_heap_store_tuple(slot, tuple, buffer, false);

slot->tts_tableOid = tuple->t_tableOid;

return slot;
}
```

We can get the two assigned values are same by reading codes. Maybe we
should remove one?

What's more, we find that maybe we assign slot->tts_tableOid in outer
interface like table_scan_getnextslot may be better and more friendly when
we import other pluggable storage formats. It can avoid duplicated
assignments in every implementation of table AM's interfaces.

Regards,
Wenchao