Remove provider position from ws_meta and view

Provider position clutters the public interface and is not
required by wsrep-API v26. Remove the position from ws_meta/view
classes and deal with it internally in custom providers.

dbsim/db_server_service.cpp

@@ -135,8 +135,7 @@ wsrep::view db::server_service::get_view(wsrep::client_service&,
         stored_view.capabilities(),
         my_idx,
         stored_view.protocol_version(),
-        stored_view.members(),
-        0
+        stored_view.members()
     );
     return my_view;
 }