Do not continue serving variant if it was removed from the split traffic

At the moment, if you change the split ratio after an experiment ran for a while and assign 0% to one of the variants, you'll still get some traffic from users that have potentially persisted that variant in their local storage.

### Expected Behaviour

If we pull a variant from an experiment, it should not get any traffic anymore.

### Actual Behaviour

The variant still gets traffic once the choice was persisted to the local storage.

Start by tracing how the experiment choice is persisted in local storage and how the current split ratio is applied when selecting a variant. Confirm the behavior with a persisted variant assigned 0% traffic; done means that variant is no longer served after it is removed from the split.

Written by the indexing model from the issue text.