disable AES_convert_key by #if 0'ing it out (suggested by mcb30)

src/crypto/axtls/crypto.h

 void AES_cbc_encrypt(AES_CTX *ctx, const uint8_t *msg, 
         uint8_t *out, int length);
 void AES_cbc_decrypt(AES_CTX *ks, const uint8_t *in, uint8_t *out, int length);
+#if 0 /** currently unused function **/
 void AES_convert_key(AES_CTX *ctx);
+#endif
 
 /**************************************************************************
  * RC4 declarations