Don't need the second reference to extfiltr.nlm anymore now that there are...

Don't need the second reference to extfiltr.nlm anymore now that there are multiple submakefiles in the filters directory.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@290837 13f79535-47bb-0310-9956-ffa450edef68