Drop table `operations_feature_flag_scopes`

The table operations_feature_flag_scopes can be dropped, since it is no longer used.

Discussed in !147541 (merged), and also marked as ready to be dropped in it's yml:

This table currently has no data in it, and it has been verified via db-lab: Proof: Row count = 0, from db-lab.

Implementation Guide

Follow the guide in https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table.

2 migrations are needed - the first one to drop the foreign key, and the second one to drop the table itself.