Remove rogue security.md file not used anywhere

Created by: mickadoo

https://github.com/civicrm/civicrm-user-guide/blob/master/docs/initial-set-up/security.md exists in the repo, but it is not used in mkdocs.yml and cannot be viewed anywhere.

According to Sean it was moved to the sysadmin guide, but not cleaned up so it should be deleted.