Remove deprecated gitlab_main_clusterwide schema references from codebase

Description:

All tables have been migrated away from the gitlab_main_clusterwide schema. This issue tracks the removal of all remaining code references to this deprecated schema.

Background

The gitlab_main_clusterwide schema has been deprecated as part of the GitLab Cells initiative. All tables previously using this schema have been migrated to appropriate alternatives (gitlab_main_cell, gitlab_main_cell_local, gitlab_main_user, gitlab_main_cell_setting, or gitlab_main_org).

Now we need to clean up all code references to prevent confusion and ensure the schema is no longer available for use.

Steps to Complete

• Step 1: Remove schema from specs and tests
  • Find all spec files that reference gitlab_main_clusterwide (e.g., spec/lib/gitlab/database/gitlab_schema_spec.rb)
  • Remove test cases that validate or allow this schema
  • Update any test fixtures or mocks that use this schema
  • Ensure CI/CD pipeline prevents creation of new tables with this schema
  • A spec is kept to ensure no new table is added to gitlab_main_clusterwide schema and to be removed in the last step..
• Step 2: Remove from code that relies on the schema
  • Search for any application code that checks for or handles gitlab_main_clusterwide
  • Remove conditional logic that treats this schema differently
  • Update any schema validation or classification logic
  • Remove from any allow-lists or exemption lists
• Step 3: Remove from documentation and comments
  • Remove from db/docs/ schema documentation
  • Remove from code comments that reference this schema
  • Update any migration guides or developer documentation
  • Remove from schema selection guidelines
• Step 4: Remove schema constant and definition
  • Remove gitlab_main_clusterwide constant from schema definitions (likely in lib/gitlab/database/gitlab_schema.rb or similar)
  • Remove from any schema enum or list definitions
  • Remove from any schema validation constants
  • Remove the gatekeeping spec that allows no new tables to use gitlab_main_clusterwide schema
• Step 5: Verify removal is complete
  • Run grep/search to ensure no remaining references exist
  • Verify CI/CD pipeline passes
  • Confirm no broken references in documentation

Acceptance Criteria

• No references to gitlab_main_clusterwide remain in the codebase
• All tests pass
• CI/CD pipeline prevents creation of new tables with this schema
• No broken links or references in documentation

Related Issues

• #504188 (closed) - Detect mis-categorization of gitlab_schema
• #548458 (closed) - Create draft MRs for tables that were previously gitlab_main_clusterwide