Don't stop or warn if the edited file was deleted on disk

If an open  file with unsaved changes was deleted on disk, Okular
currently warns the user about it (in master) or prevents them from
saving it (in 23.04). But in both cases this is unnecessary; if the
open file was deleted on disk when the user tries to save changes,
Okular can simply save to its file path and re-create it, with no
risk of data loss or stomping on anyone else's work. So it should do
that.

CCBUG: 402017