Clear any existing image on subsequent clicks
Clear any existing image on subsequent clicks

Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/clustering.gmi

* If the user clicks on the "Clustered Heatmap" button, the system
  should remove any previously existing clustered heatmap image and
  replace it with the new image.

pjotrp/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,pjotrp/genenetwork2