Convert reference counts in crypto/

This change converts the reference counts in crypto/ to use
|CRYPTO_refcount_t|. The reference counts in |X509_PKEY| and |X509_INFO|
were never actually used and so were dropped.

Change-Id: I75d572cdac1f8c1083c482e29c9519282d7fd16c
Reviewed-on: https://boringssl-review.googlesource.com/4772
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/engine.h b/include/openssl/engine.h
index da242f6..d3d278a 100644
--- a/include/openssl/engine.h
+++ b/include/openssl/engine.h
@@ -93,7 +93,7 @@
 /* openssl_method_common_st contains the common part of all method structures.
  * This must be the first member of all method structures. */
 struct openssl_method_common_st {
-  int references;
+  int references;  /* dummy – not used. */
   char is_static;
 };