DEV: Delete old personal message settings (#21381)

Followup to e62e93f

Both enable_personal_messages and min_trust_to_send_messages
have been deprecated for a long time now, they can be deleted.

db/migrate/20230523073109_delete_old_personal_message_settings.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class DeleteOldPersonalMessageSettings < ActiveRecord::Migration[7.0]
+  def up
+    execute <<~SQL
+      DELETE FROM site_settings WHERE name = 'enable_personal_messages'
+    SQL
+
+    execute <<~SQL
+      DELETE FROM site_settings WHERE name = 'min_trust_to_send_messages'
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end