[Monitoring] Removing mappings.json from archive data is breaking tests

[chrisronline]

Related to elastic/elasticsearch#37773

In #29184, we had an issue where we tried to update our mappings.json for all our monitoring archives because Elasticsearch automatically inserts index templates for various .monitoring-* indices and these templates were conflicting with the updated mapping.json files we tried to use in #29184.

As a solution, we decided to just deleted all the mappings.json files and that seemed to work fine, but we've seen multiple bug reports (#29966, #29973) that are a result of this. Without a mappings.json file, the es archiver unload functionality does not properly delete the indices, leaving them in the functional test server for the next test suite which is causing unexpected results.

We need to restore these mappings.json and simply remove the mappings: { } section from each index in order to restore the es archiver unload delete functionality.

[elasticmachine]

Pinging @elastic/stack-monitoring

[chrisronline]

cc @chrisdavies