Remove double require of core factories from rails_helper

It shouldn't create big issues but that file is currently required
twice. It is also part of `solidus_dev_support/testing_support/factories`
which is required a couple of lines below.

Co-authored-by: Rainer Dema <rainerdema@users.noreply.github.com>

Author: kennyadsl

lib/solidus_dev_support/rspec/rails_helper.rb

@@ -16,7 +16,6 @@
 require 'ffaker'
 
 require 'spree/testing_support/authorization_helpers'
-require 'spree/testing_support/factories'
 require 'spree/testing_support/url_helpers'
 require 'spree/testing_support/preferences'
 require 'spree/testing_support/controller_requests'