Remove Repository Stats API (elastic#62309)

Now the Repository Stats API is deprecated in 7.10.0 (elastic#62297) 
we can remove it in 8.0.0.

I'm labeling this as >breaking to respect the development 
process but we're talking about an experimental API that 
was never released (behind a feature flag).

Author: tlrx

x-pack/plugin/searchable-snapshots/qa/rest/src/test/resources/rest-api-spec/test/repository_stats.yml

@@ -56,17 +56,14 @@
 import org.elasticsearch.xpack.core.action.XPackUsageFeatureAction;
 import org.elasticsearch.xpack.core.searchablesnapshots.MountSearchableSnapshotAction;
 import org.elasticsearch.xpack.searchablesnapshots.action.ClearSearchableSnapshotsCacheAction;
-import org.elasticsearch.xpack.searchablesnapshots.action.RepositoryStatsAction;
 import org.elasticsearch.xpack.searchablesnapshots.action.SearchableSnapshotsStatsAction;
 import org.elasticsearch.xpack.searchablesnapshots.action.TransportClearSearchableSnapshotsCacheAction;
 import org.elasticsearch.xpack.searchablesnapshots.action.TransportMountSearchableSnapshotAction;
-import org.elasticsearch.xpack.searchablesnapshots.action.TransportRepositoryStatsAction;
 import org.elasticsearch.xpack.searchablesnapshots.action.TransportSearchableSnapshotsStatsAction;
 import org.elasticsearch.xpack.searchablesnapshots.cache.CacheService;
 import org.elasticsearch.xpack.searchablesnapshots.cache.NodeEnvironmentCacheCleaner;
 import org.elasticsearch.xpack.searchablesnapshots.rest.RestClearSearchableSnapshotsCacheAction;
 import org.elasticsearch.xpack.searchablesnapshots.rest.RestMountSearchableSnapshotAction;
-import org.elasticsearch.xpack.searchablesnapshots.rest.RestRepositoryStatsAction;
 import org.elasticsearch.xpack.searchablesnapshots.rest.RestSearchableSnapshotsStatsAction;
 
 import java.util.Collection;
@@ -265,7 +262,6 @@ public Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
             new ActionHandler<>(SearchableSnapshotsStatsAction.INSTANCE, TransportSearchableSnapshotsStatsAction.class),
             new ActionHandler<>(ClearSearchableSnapshotsCacheAction.INSTANCE, TransportClearSearchableSnapshotsCacheAction.class),
             new ActionHandler<>(MountSearchableSnapshotAction.INSTANCE, TransportMountSearchableSnapshotAction.class),
-            new ActionHandler<>(RepositoryStatsAction.INSTANCE, TransportRepositoryStatsAction.class),
             new ActionHandler<>(XPackUsageFeatureAction.SEARCHABLE_SNAPSHOTS, SearchableSnapshotsUsageTransportAction.class),
             new ActionHandler<>(XPackInfoFeatureAction.SEARCHABLE_SNAPSHOTS, SearchableSnapshotsInfoTransportAction.class)
         );
@@ -283,8 +279,7 @@ public List<RestHandler> getRestHandlers(
         return List.of(
             new RestSearchableSnapshotsStatsAction(),
             new RestClearSearchableSnapshotsCacheAction(),
-            new RestMountSearchableSnapshotAction(),
-            new RestRepositoryStatsAction()
+            new RestMountSearchableSnapshotAction()
         );
     }