Removing .includes() call from MigrateTaxCategoriesToLineItems

Addresses the bug discussed in https://github.com/spree/spree/issues/4460

Note that this will make the migration slower, but less buggy in the case that LineItems refer to deleted Variants or Products.

If the `acts_as_paranoid` gem can be modified to allow "unscoped" `includes()` calls, then we can reinstate the performance improvement.

Fixes #4460
Fixes #4471

Conflicts:
	core/db/migrate/20130802022321_migrate_tax_categories_to_line_items.rb