bookmark deletion may lead to duplicated bookmarks

[diracdeltas]

this was caused by trying to handle deletes in an appStoreChange listener as part of #9349. it turns out this does not work reliably.

STR:

1. set up sync on two pyramids
2. in pyramid 0, create two folders. bookmark a site and move it into a folder. wait for the change to propagate.
3. move the site from one folder to another in pyramid 0.
4. in pyramid 1, the site may appear in both folders instead of only in the new folder.