DEV: Remove old enable_experimental_composer_uploader site setting (#15406)

No longer used, the uppy method is now the default for
composer uploads and the old code is deleted.

db/migrate/20211224011511_delete_experimental_composer_upload_setting.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class DeleteExperimentalComposerUploadSetting < ActiveRecord::Migration[6.1]
+  def up
+    execute "DELETE FROM site_settings WHERE name = 'enable_experimental_composer_uploader'"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end