Remove potential double free

Janis noticed that GetAuthTokenKey has ownership retained by the
implementer, but the caller in libgatekeeper is delete[]ing it.

Bug: 120610663
Bug: 129768470
Test: gatekeeper-unit-tests, Gatekeeper 1.0 VTS against Trusty
Change-Id: I67171d707706b1ebe5caad47447762144495e28b
(cherry picked from commit 6d2ee18a9af8eb9d8580850e434d21e4481fa36e)
(cherry picked from commit 303e6db4d5fbdd42be5bfda1419c759f8e8e4626)
diff --git a/gatekeeper.cpp b/gatekeeper.cpp
index 44993cf..607da7a 100644
--- a/gatekeeper.cpp
+++ b/gatekeeper.cpp
@@ -235,7 +235,6 @@
         uint32_t hash_len = (uint32_t)((uint8_t *)&token->hmac - (uint8_t *)token);
         ComputeSignature(token->hmac, sizeof(token->hmac), auth_token_key, key_len,
                 reinterpret_cast<uint8_t *>(token), hash_len);
-        delete[] auth_token_key;
     } else {
         memset(token->hmac, 0, sizeof(token->hmac));