gpgsm: Cleanup of legacy variable name use.

* sm/encrypt.c (gpgsm_encrypt): Unify use of RC and ERR.
* sm/sign.c (gpgsm_sign): ditto.

Initially we didn't used the gpg_error_t thingy and while migrating
we sometimes used RC and ERR for tracking the error. This is pretty
error prone and thus we better remove it (after 20 years).