Discarding an activity does not discard its mirrored bookmark

Seems easy enough to reproduce. Create an activity, wait for the time interval for bookmarks to be saved and also activities to persist. Discard an activity. The bookmark remains behind.

Shite, sorry. While I am writing this, I took a look at the source code for ```activities-discard```, it says:

```elisp
;; TODO: Discard relevant bookmarks when `activities-bookmark-store' is enabled.
```

I guess this issue can stand as a reminder to contributors to write that code. It could be me if I can learn the necessary elisp skills.