Remove 2 unnecessary indexes related to traversal ids

Problem to solve

In an MR, @ahegyi suggested creating a follow-up issue to remove 2 indexes.

Proposal

Remove these 2 database indexes:

1. tmp_index_namespaces_empty_traversal_ids_with_root_namespaces
2. tmp_index_namespaces_empty_traversal_ids_with_child_namespaces

Note: No deprecation announcement is necessary, because this indexes are not user-facing in any way.