Remove instance variable declaration

This is not necessary and will keep an instance around
in memory for longer than we need

app/indexers/concerns/iiif_print/child_indexer.rb

@@ -17,25 +17,27 @@ def self.decorate_work_types!
         next unless work_type.iiif_print_config?
 
         if IiifPrint.use_valkyrie?(work_type)
-          decorate_valkyrie_resource(work_type)
+          indexer = decorate_valkyrie_resource(work_type)
         else
-          decorate_af_object(work_type)
+          indexer = decorate_af_object(work_type)
         end
 
-        @indexer.prepend(self).class_attribute(:iiif_print_lineage_service, default: IiifPrint::LineageService) unless @indexer.respond_to?(:iiif_print_lineage_service)
+        indexer.prepend(self).class_attribute(:iiif_print_lineage_service, default: IiifPrint::LineageService) unless indexer.respond_to?(:iiif_print_lineage_service)
         work_type::GeneratedResourceSchema.send(:include, IiifPrint::SetChildFlag) if work_type.const_defined?(:GeneratedResourceSchema)
       end
     end
 
     def self.decorate_valkyrie_resource(work_type)
       work_type.send(:include, Hyrax::Schema(:child_works_from_pdf_splitting)) unless work_type.included_modules.include?(Hyrax::Schema(:child_works_from_pdf_splitting))
-      @indexer = "#{work_type.to_s}Indexer".constantize
-      @indexer.send(:include, Hyrax::Indexer(:child_works_from_pdf_splitting)) unless @indexer.included_modules.include?(Hyrax::Indexer(:child_works_from_pdf_splitting))
+      # TODO: Use `Hyrax::ValkyrieIndexer.indexer_class_for` once changes are merged.
+      indexer = "#{work_type.to_s}Indexer".constantize
+      indexer.send(:include, Hyrax::Indexer(:child_works_from_pdf_splitting)) unless indexer.included_modules.include?(Hyrax::Indexer(:child_works_from_pdf_splitting))
+      indexer
     end
 
     def self.decorate_af_object(work_type)
       work_type.send(:include, IiifPrint::SetChildFlag) unless work_type.included_modules.include?(IiifPrint::SetChildFlag)
-      @indexer = work_type.indexer
+      work_type.indexer
     end
 
     def to_solr