cryptodev: use NULL throttle timer cb for read direction

Operations on a cryptodev are considered as *write* only, the callback
of read direction is never invoked. Use NULL instead of an unreachable
path(cryptodev_backend_throttle_timer_cb on read direction).

The dummy read timer(never invoked) is already removed here, it means
that the 'FIXME' tag is no longer needed.

Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Message-Id: <20230728022006.1098509-6-pizhenwei@bytedance.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

backends/cryptodev.c

@@ -341,8 +341,7 @@ static void cryptodev_backend_set_throttle(CryptoDevBackend *backend, int field,
     if (!enabled) {
         throttle_init(&backend->ts);
         throttle_timers_init(&backend->tt, qemu_get_aio_context(),
-                             QEMU_CLOCK_REALTIME,
-                             cryptodev_backend_throttle_timer_cb, /* FIXME */
+                             QEMU_CLOCK_REALTIME, NULL,
                              cryptodev_backend_throttle_timer_cb, backend);
     }