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

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.

db/post_migrate/20210923135909_remove_delayed_project_removal_from_namespaces.rb

+# frozen_string_literal: true
+
+class RemoveDelayedProjectRemovalFromNamespaces < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+  enable_lock_retries!
+
+  def up
+    remove_column :namespaces, :delayed_project_removal
+  end
+
+  def down
+    add_column :namespaces, :delayed_project_removal, :boolean, default: false, null: false # rubocop:disable Migration/AddColumnsToWideTables
+    add_concurrent_index :namespaces, :id, name: 'tmp_idx_on_namespaces_delayed_project_removal', where: 'delayed_project_removal = TRUE'
+  end
+end