Suggest we remove complicated must-statement on port list

[cmoberg]

There is a fairly complicated must-constraint on the port list that I suggest we remove. It depends on operational data (e.g. the current rate of the port) which is a complicated and thorny issue.

The must statement is here: https://github.com/OpenNetworkingFoundation/configuration/blob/master/drafts/of-config.yang#L398