Removed redundant $alternate_base_url switching

The `$alternate_base_url` switching code below doesn't seem to serve a purpose. I'm sure it was probably put there for a reason so please correct me if I'm wrong - but I couldn't find any positive impact it had on the generated cache files. It does however have some negative impacts:

* causes all links in the generated page to become absolute, which is not necessary because of the included base tag, and also makes it inconsistent with the live site
* documentation states that `FilesystemPublisher::$static_base_url` must not include a trailing slash, but omitting this in combination with the absolute link conversion causes all paths to assets to be broken (e.g. mysite.comthemes/css/etc.)
* causes some mangling of links that point to pages on other subsites

Fixes #15