Remove MultipleDatabases offenses from todo

Remove old Database/MultipleDatabases offenses from the rubocop todo
file that are no longer needed. In these cases, the offending code has
either been fixed or removed.

.rubocop_manual_todo.yml

@@ -2400,14 +2400,9 @@ Database/MultipleDatabases:
     - 'lib/gitlab/database/load_balancing/sticking.rb'
     - 'lib/gitlab/database/migrations/observers/migration_observer.rb'
     - 'lib/gitlab/database/migrations/observers/query_log.rb'
-    - 'lib/gitlab/database/multi_threaded_migration.rb'
     - 'lib/gitlab/database/partitioning_migration_helpers/backfill_partitioned_table.rb'
-    - 'lib/gitlab/database/postgresql_adapter/dump_schema_versions_mixin.rb'
-    - 'lib/gitlab/database/postgresql_database_tasks/load_schema_versions_mixin.rb'
     - 'lib/gitlab/database.rb'
     - 'lib/gitlab/database/schema_cache_with_renamed_table.rb'
-    - 'lib/gitlab/database/schema_migrations/context.rb'
-    - 'lib/gitlab/database/schema_version_files.rb'
     - 'lib/gitlab/database/with_lock_retries.rb'
     - 'lib/gitlab/gitlab_import/importer.rb'
     - 'lib/gitlab/health_checks/db_check.rb'