flatpak: flatpak-builder package should be enough

We need to do one at a time, so we can default to flatpak-builder which
will require flatpak too.

plugins/flatpak/gbp-flatpak-workbench-addin.c

@@ -219,7 +219,6 @@ gbp_flatpak_workbench_addin_install_flatpak_builder (GSimpleAction *action,
   IdeContext *context;
 
   static const gchar *packages[] = {
-    "flatpak",
     "flatpak-builder",
     NULL
   };