Remove ctx_new and ctx_free function pointers

Now that HMAC isn't special anymore, and with the help of init params,
this isn't needed anymore as we only allocate memory for the state.

This effectively reverts e9bcc64e62 .

auth_hmac.c

@@ -114,12 +114,6 @@ hmac_auth(uint8_t *out, size_t *outlen, void *initparams, const uint8_t *in,
 	    hmac_final(&ctx, out, outlen);
 }
 
-static void *
-hmac_ctx_new(void)
-{
-	return malloc(sizeof(struct hmac_ctx));
-}
-
 
 static struct lc_auth_impl	hmac_impl = {
 	.init = &hmac_init,
@@ -127,9 +121,7 @@ static struct lc_auth_impl	hmac_impl = {
 	.final = &hmac_final,
 	.auth = &hmac_auth,
 
-	.ctx_new = &hmac_ctx_new,
-	.ctx_free = NULL,
-
+	.argsz = sizeof(struct hmac_ctx),
 	.blocklen = 0,
 	.taglen = 0,
 };