Plots: Update legend suppression (smooth template)

[mattseddon]

Now that the default template is a smooth plot our (flaky) method of suppressing the legend and then reinstating it in a zoomed plot no longer works as expected/produces a nice result:

We need to decide whether or not we are going to continue attempting to display the legend/data in the old way or if we are going to take a different approach. If we are going to change then we need suggestions for how to integrate the sidebar/rest of the app.

After deciding on the approach we will need to spend time updating the implementation.

Note: I have spent some time looking into a fix/patch for the change and it appears that the change will be non-trivial. I think we are going to have to start walking through the template hierarchy to update the different color/strokedash/shape entries.

[mattseddon]

Related to #3223

[shcheklein]

It should be fixed/researched along with iterative/dvc-render#115

[shcheklein]

Looking into this. I'll try to remove the legend objects from the Spec as we do with titles.

[shcheklein]

Closing for now in favor of iterative/dvc-render#115 (we can reopen if @dberenbaum decides other way).