One of them is used in the new minimal SSL BIO, but cURL doesn't consume
it, so let's just leave it out. A consumer using asynchronous
certificate lookup is unlikely to be doing anything with SSL BIOs.

Change-Id: I10e7bfd643d3a531d42a96a8d675611d13722bd2
Reviewed-on: https://boringssl-review.googlesource.com/12686
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>