Drop the `trending_projects` table

See #553408 (closed) for context

Step 3 of removing TrendingProjectsWorker. Drops the trending_projects table and its TrendingProject ActiveRecord model now that nothing writes to or reads from it.

Reference: https://docs.gitlab.com/development/database/avoiding_downtime_in_migrations/#dropping-tables

Prerequisite: #601483 (closed) must be merged and deployed.

Implementation Steps

1. Generate a post-deployment migration to drop the table:

   rails g post_deployment_migration drop_trending_projects_table

   In the migration, use drop_table :trending_projects.

2. Delete app/models/trending_project.rb.

3. Delete spec/models/trending_project_spec.rb (if it exists).

4. Search for any remaining references to TrendingProject or trending_projects and remove them (e.g. factory files, index definitions in schema).