Multiple labels showing up on LabelMap popups

[misaugstad]

Brief description of problem/feature

In #3549 @JeremyFreiburger fixed an issue with LabelMap essentially crashing after you look at too many labels. Unfortunately, it looks like part of this fix involved previous labels not being removed from GSV when opening subsequent labels! So you can end up with multiple labels on the image if you look at multiple from the same pano.