Remove gitlab_main_clusterwide from application code (Step 2)

What does this MR do and why?

Step 2 of removing deprecated gitlab_main_clusterwide schema references from the codebase.

This MR removes gitlab_main_clusterwide from application code that relies on the schema:

• Removes Danger plugin (tooling/danger/gitlab_schema_validation_suggestion.rb and danger/plugins/gitlab_schema_validation.rb) - This plugin warned developers about using gitlab_main_clusterwide. Since the schema is deprecated and no longer valid, this warning is no longer needed.

• Removes from query analyzer schema mapping (lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb) - Removes gitlab_main_clusterwide: :gitlab_main from SCHEMA_MAPPING since this schema no longer exists.

• Removes test table prefix mapping (lib/gitlab/database/gitlab_schema.rb) - Removes _test_gitlab_main_clusterwide_ from IMPLICIT_GITLAB_SCHEMAS.

References

• Parent issue: #583699
• Step 1 MR: !215178

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.