gpg: Avoid duplicate declaration of {no-,}sk-comments noops.

* g10/gpg.c: Cleanup argument parsing.

With c76117f8b0165fe5cec5e7f234f55f5a4cd7f0ab, the GnuPG 2.0.x branch
accidentally introduced a second (identical) argument parser for both
--sk-comments, and for --no-sk-comments.

This caused short versions (e.g. omitting the trailing "s", as gpgme
does) of either command to fail with:

gpg: option "--sk-comment" is ambiguous