BulkImports: Remove unused `after_run` DSL

On https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51595, the
`after_run` DSL was deprecated in favor of using `after_run` methods in
the Pipelines. For this reason, now we don't need the `after_run` DSL
anymore.

lib/bulk_imports/pipeline.rb

@@ -22,10 +22,6 @@ module BulkImports
         @loaders ||= instantiate(self.class.get_loader)
       end
 
-      def after_run
-        @after_run ||= self.class.after_run_callback
-      end
-
       def pipeline
         @pipeline ||= self.class.name
       end
@@ -52,10 +48,6 @@ module BulkImports
         class_attributes[:loader] = { klass: klass, options: options }
       end
 
-      def after_run(&block)
-        class_attributes[:after_run] = block
-      end
-
       def get_extractor
         class_attributes[:extractor]
       end
@@ -68,10 +60,6 @@ module BulkImports
         class_attributes[:loader]
       end
 
-      def after_run_callback
-        class_attributes[:after_run]
-      end
-
       def abort_on_failure!
         class_attributes[:abort_on_failure] = true
       end