gpg: Do not delete any keys if --dry-run is passed.

* g10/delkey.c (do_delete_key): Don't delete the keyblock on dry runs.
Do not clear the ownertrust.  Do not let the agent delete the key.

• Co-authored-by: Matheus Afonso Martins Moreira
• Signed-off-by: Werner Koch <wk@gnupg.org>