CI passes with duplicate fields

[axw]

I was expecting these to need overwrite: true - that doesn't seem to be enforced any more - this is worrying.

Originally posted by @graphaelli in #3729

If you run apm-server export index-pattern, it'll fail with duplicated fields without overwrite: true. We should ensure this is caught when updating fields.yml.

[axw]

We're deprecating Elasticsearch setup, and will be relying on the integration package for installing Elasticsearch assets (#5914). There will be no more field inheritance from beats. Not worth fixing for the remainder of 7.x.