Remove use_field_mapping format on 7.x versions

[markov00]

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

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[nyetwurk]

Please make sure that if you fix this it retroactively applies to existing visualizations.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[timroes]

Removing this from App list, since this is a pure thing inside the top_hits AggType not related to the metrics visualization specifically. So App Arch responsibility.

[lukeelmers]

We should address #64734 at the same time as this issue

[lukeelmers]

Fixing this will also likely address #68883

[lukeelmers]

I've opened a PR to address this. At the moment the plan is to have it backported for 7.9

Please make sure that if you fix this it retroactively applies to existing visualizations.

This should not be a concern as the use_field_mapping param was never stored in visualization saved objects and was only ever added on-the-fly. So once this fix is released it should remove the deprecation warning for any visualizations using a top hits aggregation.