Quote - > Quote - To remove a library, select it, next to "Show Library". A remove button will appear.

Yes, that's what I expected (I'm of those 1% who RTFM!) - but it doesn't show. No folder - button, just the folder + one...

I had thought that also, but I figured it out. 

On the drop-down arrow next to 'Show Library' choose the runtime you want to delete.  Make sure it now says the runtime you want to delete, and not 'All'  You should now see a button just to the right to delete the library.  You don't select it in the larger pane, just in the drop-down menu.  Hope this is clear.  i sometimes word things muddily.