Remove commented x509_chain declaration

TODO indicated that it was unused, and commented. Any existing users are already broken.

Change-Id: I75ebaf3f20015845d8c81eecffe2a4dfbdbe18e8
Reviewed-on: https://boringssl-review.googlesource.com/1386
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/pem.h b/include/openssl/pem.h
index 4bd9bd0..3eaac0a 100644
--- a/include/openssl/pem.h
+++ b/include/openssl/pem.h
@@ -181,8 +181,6 @@
 	int num_recipient;
 	PEM_USER **recipient;
 
-	/* XXX(ben): don#t think this is used! 
-		STACK *x509_chain;	/ * certificate chain */
 	EVP_MD *md;		/* signature type */
 
 	int md_enc;		/* is the md encrypted or not? */