Removing incorrect context free

After deriving JSEncKey we were wrongly clearing the AES context.
We shouldn't clear the context until the function is done doing its job.
That was why JSINtKey derivation was wrong which would result in MIC
failures.

features/lorawan/lorastack/mac/LoRaMacCrypto.cpp

@@ -31,7 +31,6 @@
 #include "system/lorawan_data_structures.h"
 #include "mbedtls/platform.h"
 
-
 #if defined(MBEDTLS_CMAC_C) && defined(MBEDTLS_AES_C) && defined(MBEDTLS_CIPHER_C)
 
 LoRaMacCrypto::LoRaMacCrypto()
@@ -360,7 +359,6 @@ int LoRaMacCrypto::compute_join_server_keys(const uint8_t *key, uint32_t key_len
     if (0 != ret)
         goto exit;
 
-    mbedtls_aes_free(&aes_ctx);
     memset(nonce, 0, sizeof(nonce));
     nonce[0] = 0x06;
     memcpy(nonce + 1, eui, 8);