Everyone calls this with NULL anyway. People never actually use
lh_CONF_VALUE_* functions (or any other lh_* functions for that matter).

Also remove unused X509V3_EXT_CRL_add_conf prototype.

This removes one of the last mentions of LHASH_OF in public headers.

Update-Note: X509V3_EXT_conf_nid calls that pass a non-NULL first
    parameter will fail to compile.

Change-Id: Ia6302ef7b494efeb9b63ab75a18bc340909dcba3
Reviewed-on: https://boringssl-review.googlesource.com/32117
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>