Take thread.h out of the documentation.

There's nothing in there that isn't deprecated, since BoringSSL is thread-safe
by default now.

Change-Id: Idfd9de8bd3a6544b1d4176b2d115eef9eefa63d0
Reviewed-on: https://boringssl-review.googlesource.com/6031
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/util/doc.config b/util/doc.config
index a427e04..7c96ec7 100644
--- a/util/doc.config
+++ b/util/doc.config
@@ -16,7 +16,6 @@
       "include/openssl/obj.h",
       "include/openssl/rand.h",
       "include/openssl/stack.h",
-      "include/openssl/thread.h",
       "include/openssl/time_support.h"
     ]
   },{