Remove GDestroyNotify casts in g_clear_pointer() uses

Remove GDestroyNotify casts in g_clear_pointer() uses

GLib master propagates argument types in g_clear_pointer(), which causes
the usual function pointer casts to GDestroyNotify to trip compiler
warnings. Additionally, this commit changes some cleanup functions where
appropriate (wl_data_source_destroy ->
gtk_primary_selection_source_destroy for struct
gtk_primary_selection_source).