Revert "allowCrossProject for _search (#136397)" (#136958)

This reverts commit 88ef3dc.

Author: piergm

server/src/main/java/org/elasticsearch/action/search/SearchRequest.java

@@ -163,11 +163,6 @@ public boolean allowsRemoteIndices() {
         return true;
     }
 
-    @Override
-    public boolean allowsCrossProject() {
-        return true;
-    }
-
     /**
      * Creates a new sub-search request starting from the original search request that is provided.
      * For internal use only, allows to fork a search request into multiple search requests that will be executed independently.