`_metasets` are not synced when dataset is removed

[kurkle]

Possible Solution

Add event listeners to datasets array

Steps to Reproduce (for bugs)

https://www.chartjs.org/samples/master/charts/line/basic.html
Click removeDataset, click addData.
(Error in console)

Environment

• Chart.js version: master

[benmccann]

Potentially because of #6778

I don't think we'd need to add event listeners to the datasets array. Maybe instead we could add a reference to the corresponding dataset on the metaset and use that to keep them in sync when update is called