Remove dashboards, remove config options and docs for index patterns,

[simitt]

We discussed that we will remove everything Kibana and dashboard related from apm-server.yml and the docs, as the preferred way of loading dashboards and index patterns is via Kibana.

Since the index pattern still needs to be created from libbeat logic, and the UI solution doesn't allow for configuration yet, we will still keep the functionality around for expert users but clean up the configuration file.

docs changes: #1715

UPDATE:
We will remove the documentation and config options around Kibana index pattern and dashboards.
We will remove bundling dashboards with Kibana and APM Server.
(updated the title)

[simitt]

Update: We should completely remove the functionality to import dashboards from the APM Server and remove bundling the dashboards with APM Server and Kibana for 7.0.

[simitt]

@LeeDr please note that the visualisations and dashboards will be completely removed from bundling with the APM Server and Kibana, once this is merged.