remove unnecessary empty build.gradle files

these have no impact, as the targets are added by the parent
`build.gradle`. thus they do not need to exist and are currently just
cluttering the workspace and confusing people trying to work in that
area.

see also the discussion on opensearch-build#99 for some details.

Signed-off-by: Ralph Ursprung <Ralph.Ursprung@avaloq.com>

Author: rursprung

CHANGELOG.md

@@ -88,6 +88,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Remove LegacyESVersion.V_7_2_ and V_7_3_ Constants ([#4702](https://github.com/opensearch-project/OpenSearch/pull/4702))
 - Always auto release the flood stage block ([#4703](https://github.com/opensearch-project/OpenSearch/pull/4703))
 - Remove LegacyESVersion.V_7_4_ and V_7_5_ Constants ([#4704](https://github.com/opensearch-project/OpenSearch/pull/4704))
+- Removed unnecessary empty `build.gradle` files ([#4723](https://github.com/opensearch-project/OpenSearch/pull/4723))
 
 ### Fixed