[es6] ES doesn't support multiple types in a single index: use `_doc` type for all

cubicweb_elasticsearch/entities.py

@@ -127,7 +127,7 @@ class IFullTextIndexSerializable(view.EntityAdapter):
 
     @property
     def es_doc_type(self):
-        return self.entity.cw_etype
+        return '_doc'
 
     @cachedproperty
     def fulltext_indexable_attributes(self):