[Lens] Resetting a layer should clear suggestions

[wylieconlon]

Current behavior:

1. Configure a visualization in Lens, any way that you like
2. Select a suggestion
3. Click the "reset layer" button
4. Suggestions are still shown, even though the visualization is empty

The last step is unexpected, as described here.

Most likely, the best user experience here is to update suggestions when layers are changed. This is an easy change, and probably what's expected.

There is an alternative way of solving this problem, which is to make suggestions read-only until they are confirmed. Users would then understand that suggestions are a starting point for further editing.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)