Cleanup unused tagging records

What does this MR do and why?

Deletes unused records from the database. These are not used since we moved to labels 7 years ago: d45a6b29

This deletes 53855 issue taggings and 125 merge request taggings.

Migration output

== 20211028100303 TmpIndexForDeleteIssueMergeRequestTaggingsRecords: migrating
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:taggings, :id, {:where=>"taggable_type IN ('Issue', 'MergeRequest')", :name=>"tmp_index_taggings_on_id_where_taggable_type_issue_mr", :algorithm=>:concurrently})
   -> 0.0043s
-- add_index(:taggings, :id, {:where=>"taggable_type IN ('Issue', 'MergeRequest')", :name=>"tmp_index_taggings_on_id_where_taggable_type_issue_mr", :algorithm=>:concurrently})
   -> 0.0101s
== 20211028100303 TmpIndexForDeleteIssueMergeRequestTaggingsRecords: migrated (0.0223s)

== 20211028100843 DeleteIssueMergeRequestTaggingsRecords: migrating ===========
-- transaction_open?()
   -> 0.0000s
-- indexes(:taggings)
   -> 0.0041s
-- remove_index(:taggings, {:algorithm=>:concurrently, :name=>"tmp_index_taggings_on_id_where_taggable_type_issue_mr"})
   -> 0.0043s
== 20211028100843 DeleteIssueMergeRequestTaggingsRecords: migrated (0.0258s) ==

== 20211028100843 DeleteIssueMergeRequestTaggingsRecords: reverting ===========
== 20211028100843 DeleteIssueMergeRequestTaggingsRecords: reverted (0.0000s) ==

== 20211028100303 TmpIndexForDeleteIssueMergeRequestTaggingsRecords: reverting
-- transaction_open?()
   -> 0.0000s
-- indexes(:taggings)
   -> 0.0034s
-- current_schema()
   -> 0.0003s
== 20211028100303 TmpIndexForDeleteIssueMergeRequestTaggingsRecords: reverted (0.0063s)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #335794 (closed)