Remove caching as now all sitemap content is stored in database

It appears the Drupal layer of caching is not necessary now that the sitemap index is also stored in the database as opposed to being generated on the fly. There are just a few database calls on every sitemap view which seems to have little impact. This is just theoretical, as I have not measured the performance hit of calling the sitemap without caching as opposed to the cached variant.

If anyone wants to do some testing and support the idea of caching the 3.x version of the module, I am willing to look at the data and possibly reimplement caching.