gpg: Print a note if no args are given to --delete-key

It is a bit surprising that nothing happens if no key is specified to
--delete-key et al. Although this is common Unix behaviour the use
might have expected that it behaves like --export and deletes all
keys. Sure we don't do the latter, so a short notice will help.

• GnuPG-bug-id: T4959
• Signed-off-by: Werner Koch <wk@gnupg.org>