[exporter/elasticsearch] Remove host.os.type encoding in ECS mode

[isaacaflores2]

Description

The host.os.type enrichment has been moved the processor/elasticapmprocessor for two reasons:

1. The host.os.type is only needed for events that are ingested by the OTLP receiver (not APM events ingested by the receiver/elasticapmintakereciver)
2. This type of enrichment should belong to the processor/elasticapmprocessor

Related to elastic/opentelemetry-collector-components#1069 and elastic/opentelemetry-collector-components#1087

Link to tracking issue

Fixes

Testing

1. Updated test cases
2. Removed unnecessary test

Documentation

Update README.md

[lahsivjar]

Mostly LGTM! Just a concern over the change type.

[otelbot]

Thank you for your contribution @isaacaflores2! 🎉 We would like to hear from you about your experience contributing to OpenTelemetry by taking a few minutes to fill out this survey. If you are getting started contributing, you can also join the CNCF Slack channel #opentelemetry-new-contributors to ask for guidance and get help.