only end notification observations of existing guideline objects
only end notification observations of existing guideline objects

this fixes an ignored exception (all executions in `__del__` are
ignored) dont load guidelines just to end the self observation directly

```
Exception ufoLib.UFOLibError: UFOLibError(u'metainfo.plist is missing
in /.../MyFont-Regular.otf. This file is required',) in <bound method
DoodleFont.__del__ of <lib.fontObjects.doodleFont.DoodleFont object at
0x10beb14d0>> ignored
```

typesupply/defcon,moyogo/defcon,anthrotype/defcon