From the sourcemap config docs:
Configure the Elasticsearch source map retrieval location, taking the same options as output.elasticsearch. This must be set when using an output other than Elasticsearch, and that output is writing to Elasticsearch. Otherwise leave this section empty.
This means that if you use a different output than elasticsearch, you can still configure elasticsearch in apm-server.rum.source_mapping.elasticsearch.*.
So that would mean you configure something like:
apm-server.rum.source_mapping.elasticsearch:
hosts: ["localhost:9200"]
username: "foo"
password: "changeme"
output.elasticsearch.enabled: false
output.kafka.enabled: true