[Observability metrics reference] Remove duplicated file (#669)

This PR removes
https://www.elastic.co/guide/en/serverless/current/observability-metrics-reference.html
as it duplicates
https://github.com/elastic/docs-content/tree/main/reference/data-analysis/observability/index.md


Relates to elastic/docs-projects#432.

Author: alaudazzi

reference/data-analysis/observability/index.md

@@ -1,6 +1,7 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/metrics-reference.html
+  - https://www.elastic.co/guide/en/serverless/current/observability-metrics-reference.html
 ---
 
 # Metrics reference [metrics-reference]

reference/data-analysis/observability/metrics-reference-serverless.md

@@ -255,7 +255,6 @@ toc:
           - file: data-analysis/machine-learning/ml-sum-functions.md
           - file: data-analysis/machine-learning/ml-time-functions.md
       - file: data-analysis/observability/index.md
-      - file: data-analysis/observability/metrics-reference-serverless.md
         children:
           - file: data-analysis/observability/observability-host-metrics-serverless.md
           - file: data-analysis/observability/observability-container-metrics-serverless.md