gpg: Simplify the pk_encrypt function interface.

* g10/pkglue.c (pk_encrypt): Remove superfluous arguments and reanem
variable rc to err.
* g10/encrypt.c (write_pubkey_enc): Adjust for this change.

We used to pass PK as well as information which could be taken
directly from PK. Using ERR instead of RC is just for more uniform
naming of variables.