view-model: Remove unused function

The only use of nautilus_view_model_set_items() has been replaced with
nautilus_view_model_add_items().

Related: https://bugzilla.gnome.org/show_bug.cgi?id=795307
1 job for master in 7 minutes and 31 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #28645
flatpak

00:07:31