Drop search_namespace_index_assignments and search_indices tables
Description
This drops the search_namespace_index_assignments and search_indices tables as we don't use them. Both tables have no records (see for details: #473381 (comment 2009636118) and #473381 (comment 2172991466)).
We previously removed the code related to these tables in !169909 (merged).
It reverts database migrations introduced with:
Related to #473381 (closed).
Edited by Arturo Herrero