This fixes the following issue:
1. Import a file to a new track
2. Delete the track
3. Editor Source list: select imported file,
-> Remove the selected source
-> Yes, remove the Regions and Sources
The Track and Region no longer exist in the Editor,
only the playlist still exists!
The sources to be deleted are found when iterating over whole-file
regions in `EditorSources::remove_selected_sources`. Also
RegionFactory::get_regions_using_source correctly returns
regions, _editor->remove_regions does nothing since there
is no RegionView for the given region(s). This then cashes in
```
libs/ardour/playlist.cc:2400: virtual XMLNode& ARDOUR::Playlist::state(bool) const: Assertion `r->sources ().size () > 0 && r->master_sources ().size () > 0' failed.
```
f58f5bef55