Remove unused files (elastic#3756)

Remove unused release notes files. These were created when we were
planning to nest all elasticsearch client release notes in a subsection
and the APM agent release notes in another subsection, but they are
currently not used in the assembled build.

cc @Mpdreamz

Co-authored-by: Fabrizio Ferri-Benedetti <[email protected]>

Author: colleenmcginnis

release-notes/apm-agents/index.md

@@ -1,8 +1,6 @@
 toc:
   - file: index.md
   - toc: intro
-  - toc: elasticsearch-clients
   - toc: elastic-observability
-  - toc: apm-agents
   - toc: elastic-security
   - toc: elastic-cloud-serverless