Remove relationship event attribute key prefix

[leobragaz]

As we are doing for the user_block event attribute key prefix, we can also remove the relationship's one.

{
  "type": "relationship_created",
  "attributes": [
    {
      "key": "relationship_sender",
      "value": "desmos1ht80gzw6fz6335uzsjdct6dc566aufv24quvtp"
    },
    {
      "key": "relationship_receiver",
      "value": "desmos1w7h4qvk2w50zy6wj9wuscwrvvj4jv7jtdyaqzz"
    },
    {
      "key": "relationship_subspace",
      "value": "4e188d9c17150037d5199bbdb91ae1eb2a78a15aca04cb35530cccb81494b36e"
    }
  ]
}

Basically we only need to remove relationship_ prefix.