Remove support for mapping types

[gbanasiak]

#2024 drops support for ES 7.x, so Rally can be simplified by not supporting types.

From track specification:

{
  [..]
  "indices": [
    {
      "name": "geonames",
      "body": "index.json",
      "types": ["docs"] <--- THIS
    }
  ]
  [..]
}