serverless.oblt.yml: don't install apm integration (elastic#175853)

## Summary

We will depend on the apm-data Elasticsearch plugin for setting up index
templates and ingest pipelines. We have been testing this in serverless
dev and QA with config overrides -- this is just final step to roll it
out to all environments.

See elastic/apm-server#11529

config/serverless.oblt.yml

@@ -63,10 +63,8 @@ xpack.fleet.internal.registry.excludePackages: [
     'profiler_agent',
   ]
 
-## Required for force installation of APM Package
+## Required for force installation of integration packages
 xpack.fleet.packages:
-  - name: apm
-    version: latest
   # fleet_server package installed to publish agent metrics
   - name: fleet_server
     version: latest