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.

src/libsystemd-network/sd-dhcp6-client.c

@@ -951,8 +951,6 @@ static int client_timeout_resend(sd_event_source *s, uint64_t usec, void *userda
         assert(s);
         assert(client->event);
 
-        (void) event_source_disable(client->timeout_resend);
-
         switch (client->state) {
         case DHCP6_STATE_INFORMATION_REQUEST:
                 init_retransmit_time = DHCP6_INF_TIMEOUT;
@@ -1342,7 +1340,6 @@ static int client_set_state(sd_dhcp6_client *client, DHCP6State state) {
                 assert_not_reached();
         }
 
-        (void) event_source_disable(client->timeout_resend);
         client->retransmit_time = 0;
         client->retransmit_count = 0;