gpg: Print a hint for --batch mode and --delete-secret-key.

* g10/delkey.c: Include shareddefs.h.
(delete_keys): Print a hint.

The option --yes has some side-effects so it is not desirable to use it
automatically. The code in master (2.3) has special treatment of
confirm messages and thus this patch is only for 2.2.

• GnuPG-bug-id: T4667
• Signed-off-by: Werner Koch <wk@gnupg.org>