Remove SafelyChangeColumnDefault concern from topics model

What does this MR do and why?

Remove SafelyChangeColumnDefault concern from topics. As the default value from topics organization_id was removed, we can now, remove the SafelyChangeColumnDefault concern.

Following the process described in https://docs.gitlab.com/development/database/avoiding_downtime_in_migrations/#clean-up-the-safelychangecolumndefault-concern-in-the-next-minor-release

Follow-up from !186103 (merged)

This is part of the efforts of adding topics sharding key. Related MRs:

Step	MR
1	Add sharding key tracking issues for groups_and_projects
2	Add organization id to topics
3	Change app and api to support organization id on topics
4	Uses organization id on Topics GraphQL
5	Update Projects Finder when finding by Topics
6	Remove the organization_id column default
7	Remove SafelyChangeColumnDefault concern from topics model - this mr

Related #463254 (closed)