gpg: Improve import's repair-key duplicate signature detection.

* g10/key-check.c (key_check_all_keysigs): Factor some code out to ...
(remove_duplicate_sigs): new.
(key_check_all_keysigs): Call remove_duplicate_sigs again after
reordering.

This is a follupup for commit 26bce2f01d2029ea2b8a8dbbe36118e3c83c5cba
to cleanup the code and to add a second de-duplicate step when needed.

• GnuPG-bug-id: T3994
• Signed-off-by: Werner Koch <wk@gnupg.org>