fixtures: remove a no longer needed sort
Dicts these days preserve order, so the sort is no longer needed to
achieve determinism.
As shown by the `test_dynamic_parametrized_ordering` test, this can
change the ordering of items, but only in equivalent ways (same number
of setups/teardowns per scope), it will just respect the user's given
ordering better (hence `vxlan` items now ordered before `vlan` items
compared to the previous ordering).
