core/calendar: Replace a manual GMutexLocker with G_MUTEX_AUTO_LOCK()

This takes care of static-scan's warning about a variable being set
but never read.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/581>

src/core/gcal-calendar.c

@@ -130,7 +130,6 @@ update_color (GcalCalendar *self)
 static gboolean
 update_idle_cb (gpointer data)
 {
-  g_autoptr (GMutexLocker) locker = NULL;
   GcalCalendarPrivate *priv;
   GcalCalendar *self;
 
@@ -141,7 +140,7 @@ update_idle_cb (gpointer data)
   self = GCAL_CALENDAR (data);
   priv = gcal_calendar_get_instance_private (self);
 
-  locker = g_mutex_locker_new (&priv->shared.mutex);
+  G_MUTEX_AUTO_LOCK (&priv->shared.mutex, locker);
 
   if (priv->shared.name_changed)
     {