[Lens] Do not keep label if both operation and field changes

[flash1293]

At the moment the custom label a user specified is respected and kept for most operations the user does. However in some situations this is not a good default, because the custom label is unlikely so still properly reflect the meaning of a dimension.

If a user is changing both used operation and field in a single operation (e.g. by dragging a field to a dimension which is not compatible with the current operation), there is no reason to assume the user specified label still makes sense. In this situation the default label should overwrite the custom label.

[elasticmachine]

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