gpg: Also delete key-binding signature when deleting a subkey.
* g10/delkey.c (do_delete_key): Simplify and correct subkey deletion.
- GnuPG-bug-id: T4665, T4457
- Fixes-commit: d9b31d3a20b89a5ad7e9a2158b6da63a9a37fa8a
- Signed-off-by: Werner Koch <wk@gnupg.org>