Use CallsManager in MainWindow and NewCallBox

This removes the status message at the end of the NewCallBox, since it
will be replaced by a permanent InfoBar.

As a side effect this removes also the disconnect message when a call
ends.
Fixes: https://source.puri.sm/Librem5/calls/issues/119

src/calls-application.c

@@ -346,7 +346,6 @@ start_proper (CallsApplication  *self)
 
   self->main_window = calls_main_window_new
     (gtk_app,
-     provider,
      G_LIST_MODEL (self->record_store),
      self->contacts);
   g_assert (self->main_window != NULL);