Removes applies_to tag for Serverless on Watcher page (#1951)

It seems that the tag was added by mistake as according to the
[comparison
page](https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings#elasticsearch),
it's not avaialble in Serverless.

Author: yetanothertw

explore-analyze/alerts-cases/watcher/enable-watcher.md

@@ -3,7 +3,6 @@ mapped_pages:
   - https://www.elastic.co/guide/en/cloud/current/ec-watcher.html
 applies_to:
   stack: ga
-  serverless: ga
 products:
   - id: cloud-hosted
 ---