Fix up pkcs8.h.

Somehow we ended up with duplicate 'Deprecated functions' sections.
PKCS12_get_key_and_certs ended up in one of them was probably an oversight.

Change-Id: Ia6d6a44132cb2730ee1f92a6bbcfa8ce168e7d08
Reviewed-on: https://boringssl-review.googlesource.com/6020
Reviewed-by: Adam Langley <agl@google.com>