cpp: Remove obsolete workaround

* lang/cpp/src/context.cpp (Context::startKeyListing,
Context::exportPublicKeys, Context::startPublicKeyExport): Remove
workaround.

gpgme_op_keylist_ext[_start] supports GPGME_KEYLIST_MODE_EXTERN
since a long time. And the export doesn't use a keylisting (anymore),
so that the keylisting mode is irrelevant for the export.

• GnuPG-bug-id: T5757