Protect against site start page creating redirect loop

Under the following conditions, the alias for the site start page causes the redirect facility to create a redirect loop:

• site start page is ROOT::index.adoc
• site has versioned ROOT component (e.g., component=ROOT version=6.0)
• the latest version segment is empty

Under these conditions, the ContentCatalog#registerSiteStartPage() method creates an alias where pub.url == rel.pub.url. The method should detect this case and remove the alias instead since it's unnecessary.