Remove `manage_ingest_pipelines` privilege from documentation and get builitin privileges API

`manage_ingest_pipelines` is a duplicate of `manage_pipeline`. While it is not easy to deprecate and remove this at this point, we should aim to lessen the confusion and prohibit additional usages of it by

- Remove this from the documentation
- Remove this from the response of [Get builtin privileges API](https://www.elastic.co/guide/en/elasticsearch/reference/7.6/security-api-get-builtin-privileges.html)