Remove some remnants of SSLv2 support.

Some ssl23 functions that can be folded into ssl3, declarations and macros that
don't exist anymore.

Change-Id: I8057fb0bab8b6fe7e4da7b90a4945f7f22e29cd9
Reviewed-on: https://boringssl-review.googlesource.com/1280
Reviewed-by: Adam Langley <agl@google.com>