time-zone-monitor: Switch to g_time_zone_new_identifier()

We don't even need to check for NULL here because if the system timezone
is invalid, we're too screwed to have a fallback path anyway.

src/core/gcal-time-zone-monitor.c

@@ -71,7 +71,7 @@ on_timedate1_proxy_properties_changed_cb (GcalTimeZoneMonitor *self,
   if (timezone_variant)
     timezone_identifier = g_variant_get_string (timezone_variant, NULL);
 
-  self->timezone = g_time_zone_new (timezone_identifier);
+  self->timezone = g_time_zone_new_identifier (timezone_identifier);
 
   g_debug ("System timezone is %s", g_time_zone_get_identifier (self->timezone));