Adding multiple structures in the demo causes bugs

[ihh]

In the "PFAM SARS-CoV2 multi-stockholm" example, adding more than one PDB structure causes glitchy overlaid visualizations (if the first structure is not removed before adding the second) or an empty screen/crash (if the first structure is removed before adding the second).

[cmdcolin]

I am proposing for removing the structures features from the gmod.github.io/react-msaview demo for now. I will have to re-add them back, and we can use lessons that were gleaned from the jbrowse plugin implementation