This short series removed some ancient legacy code from
migration_manager and schema_tables, before I make further changes in this area.
We have more such code under the cql3 hierarchy but it can be dealt with as a follow up.
No backport required
Closesscylladb/scylladb#19530
* github.com:scylladb/scylladb:
schema_tables: remove dead code
migration_manager: remove dead code