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: cc6069ac6ecd57dcbb808f28d54fd9f89dc55014
- Signed-off-by: Werner Koch <wk@gnupg.org>