Remove outdated “upgrade” code

We used to store selected URLs in the user defaults and added this code to remove potentially items.
Fix issue with remembering expanded file browser items

Previously each file browser would have its own array of expanded items, read from user defaults, and written back during application termination.

This meant that the last file browser to write its array, would eclipse the other file browsers.

Now we have all but the first file browser append its list of expanded items. Additionally each file browser will only append items which are actually visible.

Fixes #873.