Remove XPI and JAR MIME types from httpd config
    
    We added MIME types for JAR and XPI files, which were needed for
    correct handling of the Firefox auto-configuration plugin.  The
    plugin was removed some time ago, so remove the media type
    definitions.
    
    Part of: https://pagure.io/freeipa/issue/7226
    
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>