md_acme_drive.c: remove unused variable

Compiling in maintainer mode leads to a failure
due to challenges_configured initialized but
not used. Removing it seems harmless, Stefan
please let me know if this is not the case.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1843743 13f79535-47bb-0310-9956-ffa450edef68