Clean up schema for table taggings

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Per https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Unused field: tagger_type is always NULL
• Unused field: tagger_id is always NULL
• Missing FK: tag_id -> tags
• Duplicate data: context could be enum or 4-byte integer
• Duplicate data: taggable_type could be enum or 4-byte integer
• "Polymorphic" table prevents regular FKs, use triggers or change to independent columns or non-polymorphic tables
• Missing trigger for FK check: taggable_type = 'Project', taggable_id -> projects.id
• Missing trigger for FK check: taggable_type = 'MergeRequest', taggable_id -> merge_requests.id
• Missing trigger for FK check: taggable_type = 'Issue', taggable_id -> issues.id
• Missing trigger for FK check: taggable_type = 'Ci::Runner', taggable_id -> ci_runners.id
• Missing trigger for FK check: taggable_type = 'Ci::Job', taggable_id -> ci_builds.id (?)