Remove spurious declarations of |X509V3_EXT_conf| and friends.

These functions were #if 0'ed out in the code, which is a distraction.

Change-Id: I186196ab512565507476f9b56682bf59d003d85f
Reviewed-on: https://boringssl-review.googlesource.com/4604
Reviewed-by: Adam Langley <agl@google.com>