scd: Fix signing authentication status.
* scd/app-openpgp.c (do_sign): Clear DID_CHV1 after signing.
We have a corner case: In "not forced" situation and authenticated,
and it is changed to "forced", card implementaiton can actually accept
signing, but GnuPG requires authentication, because it is "forced".
- GnuPG-bug-id: T4177
- Signed-off-by: NIIBE Yutaka <email@example.com>