Kibana version:
7.x

Describe the bug:

When using a Metric visualization with a Top Hits metric, ES will log the following warning:

{"level": "WARN", "component": "o.e.d.s.f.s.DocValueFieldsFetchSubPhase",  "message": "[use_field_mapping] is a special format that was only used to ease the transition to 7.x. It has become the default and shouldn't be set explicitly anymore."  }

Steps to reproduce:

1. Create a Metric Visualization
2. Specify a Top Hits as metric aggregation
3. Choose a field different from timestamp
4. See the warning logged in the ES logs

Expected behavior:

No warning should be logged

Screenshots (if relevant):
n/a

Errors in browser console (if relevant):
n/a