♻️ Favor explicit method over class attribute

The `class_attribute` best serves cases where we might adjust
configuration at run-time or during initialization.  In the case of of
the class method I am removing, it's subtly different than the ideal use
case for `class_attribute`.

The difference is such:

We include the configuration module into a model; this is the run time
configuration.  And because we've "configured IIIF Print" for that
model, we always will have a `.iiif_print_config?` of true.

app/indexers/concerns/iiif_print/child_indexer.rb

@@ -13,7 +13,8 @@ def self.decorate_work_types!
       #
 
       Hyrax.config.curation_concerns.each do |work_type|
-        next unless work_type.respond_to?(:iiif_print_config)
+        next unless work_type.respond_to?(:iiif_print_config?)
+        next unless work_type.iiif_print_config?
 
         work_type.send(:include, IiifPrint::SetChildFlag) unless work_type.included_modules.include?(IiifPrint::SetChildFlag)
         indexer = work_type.indexer

lib/iiif_print.rb

@@ -131,8 +131,8 @@ def self.model_configuration(**kwargs)
     Module.new do
       extend ActiveSupport::Concern
 
-      included do
-        class_attribute :iiif_print_config, default: true
+      class_method do
+        def iiif_print_config?; true; end
       end
 
       # We don't know what you may want in your configuration, but from this gems implementation,
@@ -144,6 +144,8 @@ def self.model_configuration(**kwargs)
       define_method(:iiif_print_config) do
         @iiif_print_config ||= ModelConfig.new(**kwargs)
       end
+
+      def iiif_print_config?; true; end
     end
   end