Remove x_representation fields

[cmungall]

changes be like

{
  "id": "CHANGE:001",
  "type": "NodeRename",
  "old_value": "'nuclear envelope'",
  "new_value": "'foo bar'",
  "about_node": "GO:0005635",
  "about_node_representation": "curie",
  "@type": "NodeRename"
}

there is no need for about_node_representation if we have a prefix map for each change set.

[hrshdhgd]

It is derived from get_entity_representation() which is sprinkled all over the project. I'll need more clarification as to where it's needed and where not. This seems to be in place since a long time.