Remove authz extension (RFC5878)

Found no users of the functions which control the feature. (Also I don't
particularly want to port all of that to CBS...)

Change-Id: I55da42c44d57252bd47bdcb30431be5e6e90dc56
Reviewed-on: https://boringssl-review.googlesource.com/1061
Reviewed-by: Adam Langley <agl@google.com>