gpg: --compress-sigs and --compress-keys are not no-ops in 2.0

* g10/gpg.c: Cleanup argument parsing.

c76117f8b0165fe5cec5e7f234f55f5a4cd7f0ab mistakenly marked
compress-sigs and compress-keys as no-ops on the 2.0.x branch.

These options still have an effect on the 2.0.x branch, and the
duplicate declaration also causes the gpg argument parser to fail when
shortened versions of the option are present, like:

gpg: option "--compress-k" is ambiguous