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