HIPPLUG-1357 Remove the unused (since broken) SitemapFeedBasedOnHstSitemap and SiteMapBasedOnHstSiteMapResource sitemap generators

As a result, quite some other code was unused, which in turn triggered other code to be possible to be removed.

Note I also do a content delete for

/hst:hst/hst:configurations/hst:default/hst:components/forge-sitemap-based-on-hst-configuration-feed

since this component refers to SitemapFeedBasedOnHstSitemap.

I've tested that after a restart (with existing repository), that 'forge-sitemap-based-on-hst-configuration-feed'
component is gone, basic sitemap and NewsSitemapFeed still work as expected

The Urlset object should have to think about synchronization. It is the code
that uses the Urlset that shouldn't expect the Urlset to be thread-safe (just
like a HashSet isn't thread-safe. It make much more sense to not give all the
workers a reference to the same urlset object but all have their own

A seperate note is that I really think the worker should not extend from
Thread and a lot of the complex logic can be replaced by a simple executer service

(cherry picked from commit 031750fa)

* synchronize contains block to avoid duplicate entries

(cherry picked from commit 8213a63b)