[BUG] _type value in field of exists query returns all records

[dreamer-89]

Describe the bug
_type value for field in exists query fetches records but post type removal in #1940, _type should be considered as a random value and shouldn't return any record.

To Reproduce
Request

curl -X PUT localhost:9200/test-index

curl -X POST localhost:9200/test-index/_doc -d '{ 
    "field" : "value"
}'

curl -X POST localhost:9200/test-index/_doc -d '{ 
    "field" : "value2"
}'

curl -XPOST localhost:9200/test-index/_search?pretty -d '
{
    "query": {
        "exists": {
            "field": "_type"
         }
    }
}'

Response

{
    "took": 2,
    "timed_out": false,
    "_shards": {
        "total": 1,
        "successful": 1,
        "skipped": 0,
        "failed": 0
    },
    "hits": {
        "total": {
            "value": 4,
            "relation": "eq"
        },
        "max_score": 1.0,
        "hits": [
            {
                "_index": "test-index",
                "_id": "kvyyQ4ABr3FIIAx07N_Z",
                "_score": 1.0,
                "_source": {
                    "field": "value"
                }
            },
            {
                "_index": "test-index",
                "_id": "lfy6Q4ABr3FIIAx0k9_1",
                "_score": 1.0,
                "_source": {
                    "field": "value2"
                }
            }
        ]
    }
}

Expected behavior
_type field handled as any other random field and should return empty response.

[dreamer-89]

A straightforward fix is to have check on _type in ExistsQueryBuilder here but that would mean a separate handling for _type fields and throwing exception.

This will not be a good solution as it introduces explicit check for _type keyword. Instead of this, engine should return empty response as it does for any other random field.

[nknize]

We need to remove TypeFieldMapper since _type should no longer be supported.

[elfisher]

Should this issue still be open? It looks like this was merged #3016

[dreamer-89]

Closing this issue as bug is fixed.

There is some clean up work pending related with removing TypeFieldMapper class from code base. This cleanup PR can be raised once #3004 is merged; as it removes remaining TypeFieldMapper references from the codebase.