gpg: Delete secret key after "keytocard".

* g10/card-util.c (card_store_subkey): Add arg processed_keys.
* g10/keyedit.c (keyedit_menu): Delete secret key.

This used to work using the gpg-agent: learn we called at "save" time.
However, the recent change inhibited the creation of a shadow key by
learn if a regular key still exists. Now we do an explicit delete key
at save time. This syncs the behaviour with the description of the
man page.

• GnuPG-bug-id: T6378