Toggling off bar series, then loading data with unload: true, shows bar series again

* **C3 version**: 0.6.3 (or whatever the sample documentation uses)
* **D3 version**: v5
* **Browser**: Chrome 74
* **OS**: MacOS

I'm having an issue where if you have a bar chart with two series, and toggle one of the series to be hidden, and then load more data, the hidden series comes back, appearing on top of/or behind the other series.

You can reproduce the issue with this snippet of code I modified from one of the samples
```
var chart = c3.generate({
data: {
x: 'x',
columns: [
['x', '2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04', '2013-01-05', '2013-01-06'],
['data1', 30, 200, 100, 400, 150, 250],
['data2', 130, 340, 200, 500, 250, 350]
],
type: "bar"
},
axis: {
x: {
type: 'timeseries',
tick: {
format: '%Y-%m-%d'
}
}
}
});

setTimeout(function () {
chart.load({
unload: true,
columns: [
['x', '2013-01-01', '2013-01-02', '2013-01-03', '2013-01-04', '2013-01-05', '2013-01-06'],
['data1', 130, 300, 200, 300, 150, 250],
['data2', 130, 240, 300, 400, 250, 350]
]
});
}, 5000);
```

If you copy paste this into https://c3js.org/samples/timeseries.html

After you paste the code in, hide `data1` series by clicking it in the legend.

Wait until the load happens, and you see that the blue bar comes back.

Is this the intended behaviour?

If you remove the `unload: true` from the second code block, it doesn't happen.

Reproduce the issue in the timeseries sample using the provided c3.generate and chart.load calls, first hiding data1 and then loading with unload: true. Trace how the chart handles hidden series during that load. Done means the hidden bar series stays hidden after new data is loaded, while the existing behavior without unload: true remains unaffected.

Written by the indexing model from the issue text.