Don't allow created or isDescribedby

[jcoyne]

presently maps/DepositDRO.json has:

      "not": {"required": ["created", "isDescribedBy"]},

which indicates they are not allowed together. We need to ensure that neither field is set.

We should probably have:

      "not": {
        "anyOf": [
          { "required": ["created"] },
          { "required": ["isDescribedBy"] }
        ]
      },