[Lens] Enabling metric trend line should not enable layer deletion

[drewdaemon]

(Unreleased)

Metric trend lines works by adding a hidden layer. Our current layer deleting setup changes from clearing a layer to deleting a layer if there is more than one. In the case of metric trend lines, there are two layers, but the deleting setup should still act as though only one is present since the second is hidden.

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[stratoula]

Thing missing is the icon change. I am dropping my assignment here (I fixed the error and now it works ok) as I am going to be out next week and someone else can pick it!