Remove invalid base log message

The redfish base registry does not include a definition of this
message, which generating type-safe models has teased out.  Replace the
MutuallyExclusiveProperties message with two "PropertyValueConflict"
messages.  This seems like the closest thing, but in lieu of the things
not being in the standard, this seems like the best compromise.

Tested:
curl --insecure -X POST --user root:0penBmc
https://192.168.7.2/redfish/v1/EventService/Subscriptions

 -d
'{"MessageIds":[""],"RegistryPrefixes":[""],"Destination":"","Protocol":""}'

Returns:
{
  "error": {
    "@Message.ExtendedInfo": [
      {
        "@odata.type": "#Message.v1_1_1.Message",
        "Message": "The property 'MessageIds' could not be written because its value would conflict with the value of the 'RegistryPrefixes' property.",
        "MessageArgs": [
          "MessageIds",
          "RegistryPrefixes"
        ],
        "MessageId": "Base.1.8.1.PropertyValueConflict",
        "MessageSeverity": "Warning",
        "Resolution": "No resolution is required."
      }
    ],
    "code": "Base.1.8.1.PropertyValueConflict",
    "message": "The property 'MessageIds' could not be written because its value would conflict with the value of the 'RegistryPrefixes' property."
  }
}

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I1eaf4d06b6f5b85909392c48970e3f353af3a41e