Fix double g_object_unref() on removing last note
This error was generated each time the last note was removed: (wrapper-2.0:226820): GLib-GObject-CRITICAL **: 13:49:10.481: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
According to generated application.c, vala handles ref_counts correctly. There's no need to unref manually.
Fixes: #71 (closed)