gpg: Fix --gpgconf-list case with no conf files at all.

* g10/gpg.c (get_default_configname): Remove unused function.
(main): Provide a proper filename to gpgconf_list.

With the new option pasrer we used "UNKOWN" in this case. The problem
was that gpgconf --list-options chekcs that an absolute file is
provided and thus bails out if no config file is in /etc/gnupg or in
~/.gnupg/.

get_default_configname was not anymore in use because its function is
part of the new option parser.