Eliminate slf4j dependency.

Eliminate dependency on `slf4j` in favor of using the mediocre logging capability afforded by the Servlet 2.5 API.

This results in logging that is less awesome but less complexity in getting this Filter to work in more environments, i.e. environments not currently providing `slf4j` implementation.

In the case where the Filter actually blocks a request, it throws an Exception, and that Exception will typically be logged by upstream code in a way noticeable to the administrator regardless of this Filter having only logged the blocking message to the ServletContext log.