Tracing: Cleanup database of project_tracing_settings

Why are we doing this work

In 15.0, we disabled the Tracing features via a feature flag. After the feature flag is disabled & all application code is deleted, the database can be purged of any of the old tracing code.

⚠ Removal should take place in %15.2 or later.

Tracking issue for deprecation & removal: Monitor: Tracing Deprecation (#346540 - closed)

Relevant links

• https://docs.gitlab.com/ee/development/migration_style_guide.html#dropping-a-database-table
• https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-tables

Implementation plan

1. Drop project_tracing_settings table in a post-migration
2. clean up database dictionary records [can be done in a post-migration in same release as removes all app code]