Remove repeatitive legend of plots

[hnformentin]

Describe the bug When the focus is on the ensemble level, we should have the ensemble name as legend instead of the realization numbers.

To Reproduce Steps to reproduce the behavior:

1. Plot any response for at least two ensembles with several realizations in Plot Viewer
2. Select mode "function plot" (instead of statistics)
3. Check the legend. Is it meaningful? It also does not correspond to ert gui legend.

Expected behavior Name of ensemble as legend, in the screenshot example, default_0 and default_3.

Screenshots