Revert "Don't session-restore invalid paths"

This reverts commit c733b3aa.

There are people who prefer the old behavior of session-restoring all paths including the ones which are currently inaccessible. The idea is that Dolphin is like a web browser; if a website is not currently available, the browser tells you that instead of silently discarding the URL and showing you the homepage. Doing this can be considered a form of data loss.

In addition, the reason for the change was to prevent Dolphin from showing invalid paths on launch for unmounted removable disks, but that specific case was fixed separately by switching to ~ for any tabs/views that were showing locations on a just-unmounted removable disk. So you will never run into the problem if yo unmount a disk in Dolphin and then close it.

Accordingly, let's revert the change because it causes more problems than it resolves.

BUG: 439864 CCBUG: 427619 FIXED-IN: 21.08

The particular edge case of https://bugs.kde.org/show_bug.cgi?id=427619 can be fixed in a different way--by mounting a non-automount disk that is not already mounted when Dolphin is launched with a URL on that disk, instead of displaying an error message.