♻️ Move method and remove single instance definition

I'm unclear why we were defining methods on the conf instance;
especially given that these exist on the configuration.

With this refactor, we're favoring using the Configuration object as the
container.

lib/bulkrax.rb

@@ -33,12 +33,10 @@ module Bulkrax
   # @api public
   class Configuration
     attr_accessor :api_definition,
-                  :curation_concerns,
                   :default_field_mapping,
                   :default_work_type,
                   :export_path,
                   :field_mappings,
-                  :file_model_class,
                   :generated_metadata_mapping,
                   :import_path,
                   :multi_value_element_join_on,
@@ -97,6 +95,23 @@ def factory_class_name_coercer
       @factory_class_name_coercer || Bulkrax::FactoryClassFinder::DefaultCoercer
     end
 
+
+    def file_model_class
+      @file_model_class ||= defined?(::Hyrax) ? ::FileSet : File
+    end
+
+    def file_model_class=(val)
+      @file_model_class = val
+    end
+
+    def curation_concerns
+      @curation_concerns ||= defined?(::Hyrax) ? ::Hyrax.config.curation_concerns : []
+    end
+
+    def curation_concerns=(val)
+      @curation_concerns = val
+    end
+
     attr_writer :ingest_queue_name
     ##
     # @return [String, Proc]
@@ -222,22 +237,6 @@ def config
     conf.relationship_job_class = "Bulkrax::CreateRelationshipsJob"
     conf.required_elements = ['title']
 
-    def conf.curation_concerns
-      @curation_concerns ||= defined?(::Hyrax) ? ::Hyrax.config.curation_concerns : []
-    end
-
-    def conf.curation_concerns=(val)
-      @curation_concerns = val
-    end
-
-    def conf.file_model_class
-      @file_model_class ||= defined?(::Hyrax) ? ::FileSet : File
-    end
-
-    def conf.file_model_class=(val)
-      @file_model_class = val
-    end
-
     # Hash of Generic field_mappings for use in the view
     # There must be one field_mappings hash per view partial
     # Based on Hyrax CoreMetadata && BasicMetadata