bug#40870: Quassel IRC client trying to open links with xdg-open from a nonexisting store path

[pkill9]

From the output:

```
2020-04-26 16:45:17 [Warn ] Launch failed
(/gnu/store/jsqihy7z3bsbiixchk19dsnaj6bh9h2b-xdg-utils-1.1.3/bin/xdg-open
https://freenode.net)
```

I assume it is recording the absolute path to xdg-open somewhere when
it's first run or something, but I can't find any references to it in
the configuration directory.