system/security: remove BORINGSSL_201509 support.

The BORINGSSL_201509 define was used to make updating BoringSSL in
external/boringssl less painful. It allowed code to compile with either
the old BoringSSL (which didn't define BORINGSSL_201509) or with the new
(which does).

Now that the new version has landed, this change removes that support.

Change-Id: I19e661419f830459d015bf14e7905af2ec41b735
diff --git a/keystore-engine/android_engine.cpp b/keystore-engine/android_engine.cpp
index bb23823..d23f169 100644
--- a/keystore-engine/android_engine.cpp
+++ b/keystore-engine/android_engine.cpp
@@ -220,9 +220,7 @@
   RSA_FLAG_CACHE_PUBLIC | RSA_FLAG_OPAQUE | RSA_FLAG_EXT_PKEY,
 
   NULL /* keygen */,
-#if defined(BORINGSSL_201509)
   NULL /* multi_prime_keygen */,
-#endif
   NULL /* supports_digest */,
 };