gpg: Remove multiple subkey bindings during export-clean.
* g10/key-clean.c (clean_one_subkey_dupsigs): New. (clean_all_subkeys): Call it.
(cherry picked from commit 76989d5bd89ed11f5b3656dc4748fcfc939a46dc)
- GnuPG-bug-id: T3804
- Signed-off-by: Werner Koch <wk@gnupg.org>