Remove "cause-time" from RISC example in SSF spec

[FragLegs]

In section 10.1.3, the SSF spec shows an example RISC event:

{
  "iss": "https://idp.example.com/",
  "jti": "756E69717565206964656E746966696572",
  "iat": 1520364019,
  "aud": "636C69656E745F6964",
  "sub_id": {
    "format": "phone",
    "phone_number": "+1 206 555 0123"
  },
  "events": {
    "https://schemas.openid.net/secevent/risc/event-type/account-disabled": {
      "reason": "hijacking",
      "cause-time": 1508012752
    }
  }
}

However, cause-time is not a known property of the "account-disabled" event. We should remove that property from the example.