gtklistmodels: remove gtklistmodels polyfills

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

debian/copyright

@@ -31,19 +31,6 @@ Comment:
  From gnome-control-center,
  see https://bugzilla.gnome.org/show_bug.cgi?id=792243
 
-Files:
- src/gtklistmodels/*
-Copyright:
- 2018-2019  Benjamin Otte
- 2019       Matthias Clasen
-License: LGPL-2.1+
-
-Files:
- src/gtklistmodels/gtkprivate.h
-Copyright:
- 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
-License: LGPL-2+
-
 Files:
  data/org.gnome.Calls.metainfo.xml
 Copyright: