[Maps] blended layer aggregation error when using composite aggregation

[nreese]

Kibana 7.9-BC2

Steps to view:

1. Install flights sample data set
2. Create new map and set time range to past 6 months to next 6 months.
3. add documents layer. Keep scaling at "show clusters"
4. Style fill color by value and choose machine.os.keyword

Elasticsearch failure message

{
  "error" : {
    "root_cause" : [
      {
        "type" : "x_content_parse_exception",
        "reason" : "[23:17] [geotile_grid] unknown field [size]"
      }
    ],
    "type" : "x_content_parse_exception",
    "reason" : "[23:25] [composite] failed to parse field [sources]",
    "caused_by" : {
      "type" : "x_content_parse_exception",
      "reason" : "[23:17] [geotile_grid] unknown field [size]"
    }
  },
  "status" : 400
}

Regression caused by #70313. That PR added size to geotile_grid aggregation when nested under composite. This is not allowed.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)