configure: Remove unused CONFIGFILE

This isn't used for anything, which is just as well, because
/etc/xorg.conf is not in fact a path xserver will try to use.

Bugzilla: https://bugs.freedesktop.org/8890


Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>