Output of interpolated time series should not include the columns used to derive the result series name

For example for this query `select count(c1) from points group by time(5s), c2 into :series_name.count.[c2];` and data:

```json
[
    { "name": "points",
      "columns": ["c1", "c2", "time"],
      "points": [
        [1, "aa", 1398893416],
        [2, "aa", 1398893416],
        [3, "aa", 1398893416],
        [7, "bb", 1398893416],
        [8, "bb", 1398893416],
        [9, "bb", 1398893416]
    ]}
  ]
```

The resulting time series shouldn't have the `c2` column, since that is implied from the series name. It's more efficient to not include the same value over and over in the output time series.