Drop Delayed Project Removal column from Namespaces

What does this MR do and why?

Fixes #322611 (closed)

In !53916 (merged), delayed_project_removal was migrated to namespace_settings table and in #322610 (closed) the column was ignored.

The next step is to drop the column. This is step 2 in Dropping Columns documentation.

Database

Migrate

== 20210923135909 RemoveDelayedProjectRemovalFromNamespaces: migrating ========
-- transaction_open?()
   -> 0.0000s
-- remove_column(:namespaces, :delayed_project_removal)
   -> 0.0074s
== 20210923135909 RemoveDelayedProjectRemovalFromNamespaces: migrated (0.0519s)

Rollback

== 20210923135909 RemoveDelayedProjectRemovalFromNamespaces: reverting ========
-- transaction_open?()
   -> 0.0000s
-- add_column(:namespaces, :delayed_project_removal, :boolean, {:default=>false, :null=>false, :if_not_exists=>true})
   -> 0.0054s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:name=>"tmp_idx_on_namespaces_delayed_project_removal", :where=>"delayed_project_removal = TRUE", :algorithm=>:concurrently})
   -> 0.0151s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:namespaces, :id, {:name=>"tmp_idx_on_namespaces_delayed_project_removal", :where=>"delayed_project_removal = TRUE", :algorithm=>:concurrently})
   -> 0.0051s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20210923135909 RemoveDelayedProjectRemovalFromNamespaces: reverted (0.0598s)

MR acceptance checklist

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

db/structure.sql

@@ -26934,8 +26934,6 @@ CREATE UNIQUE INDEX term_agreements_unique_index ON term_agreements USING btree
 CREATE INDEX tmp_idx_deduplicate_vulnerability_occurrences ON vulnerability_occurrences USING btree (project_id, report_type, location_fingerprint, primary_identifier_id, id);
-CREATE INDEX tmp_idx_on_namespaces_delayed_project_removal ON namespaces USING btree (id) WHERE (delayed_project_removal = true);
-
 CREATE INDEX tmp_index_namespaces_empty_traversal_ids_with_child_namespaces ON namespaces USING btree (id) WHERE ((parent_id IS NOT NULL) AND (traversal_ids = '{}'::integer[]));
 CREATE INDEX tmp_index_namespaces_empty_traversal_ids_with_root_namespaces ON namespaces USING btree (id) WHERE ((parent_id IS NULL) AND (traversal_ids = '{}'::integer[]));