Get rid of ssl_undefined_*

The only place using it is export keying material which can do the
version check inline.

Change-Id: I1893966c130aa43fa97a6116d91bb8b04f80c6fb
Reviewed-on: https://boringssl-review.googlesource.com/4615
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/s3_lib.c b/ssl/s3_lib.c
index 0c68b48..b3562f0 100644
--- a/ssl/s3_lib.c
+++ b/ssl/s3_lib.c
@@ -488,8 +488,7 @@
     SSL3_MD_CLIENT_FINISHED_CONST, 4,
     SSL3_MD_SERVER_FINISHED_CONST, 4,
     ssl3_alert_code,
-    (int (*)(SSL *, uint8_t *, size_t, const char *, size_t, const uint8_t *,
-             size_t, int use_context)) ssl_undefined_function,
+    tls1_export_keying_material,
     0,
 };