Proxy is broken for ES due to custom headers PR

[epixa]

This is just a placeholder. I'll flesh this out with more details soon.

#8032 broke some stuff when kibana doesn't have ssl and elasticsearch does.

[alexbrasetvik]

It breaks with plain http too, seems something is rewriting http://hostname with http://1.2.3.4

[alexbrasetvik]

What has happened to preserveHost?

On a89947e:

$ git grep -i preserveH
config/kibana.yml:# elasticsearch.preserveHost: true
docs/settings.asciidoc:`elasticsearch.preserveHost` added[4.2]:: By default, the host specified in the incoming request from the browser is specified as the host in the corresponding request Kibana sends to Elasticsearch. If you set this option to `false`, Kibana uses the host specified in `elasticsearch_url`.
src/cli/serve/read_yaml_config.js:  elasticsearch_preserve_host: 'elasticsearch.preserveHost',
src/plugins/elasticsearch/index.js:        preserveHost: boolean().default(true),

It's set and documented but never used?

[epixa]

It looks like preserveHost has been broken for a year with absolutely no noise (ie. bug reports), so I'm just going to remove it entirely in 5.0.

[epixa]

This shouldn't be an issue in 5.0 because the host header won't be proxied by default anyway.

[w33ble]

Closed by #8105