sd-dhcp6-client: drop unnecessary event_source_disable()

sd-dhcp6-client: drop unnecessary event_source_disable()

The event source will be soon re-enabled in the same function.

The function client_timeout_resend() may return earlier without
re-enabling the timer souce. However,
- the timer event source is one shot by default. Hence, it is not
  necessary to disable in the callback function,
- when it returns early, then client_set_state() or client_stop() is
  called before return, and they re-ernable or disable the timer.