Fix for url_rewrite on page delete via api

For develop see PR magento#10568

Problem: when deleting a cms page via api the url_rewrites aren't deleted

Steps to reproduce:
- add cms page via api
- delete cms page via api
- add new cms page with same identifier via api
- ERROR: Could not save the page, URL key already exists

Solution: moved di.xml from etc/adminhtml to etc/ folder in CmsUrlRewrite module
Event trigger to delete url rewrites is located in di.xml