lib/chart: Use PHP's DateTime instead of our DateTimeFactory

I'm about to drop the DateTimeFactory because date_default_timezone_set() in combination w/ PHP's DateTime is sufficient.

refs #6778

library/Icinga/Chart/Unit/CalendarUnit.php

@@ -4,7 +4,7 @@
 
 namespace Icinga\Chart\Unit;
 
-use Icinga\Util\DateTimeFactory;
+use DateTime;
 
 /**
  * Calendar Axis Unit that transforms timestamps into user-readable values
@@ -81,7 +81,7 @@ class CalendarUnit extends LinearUnit
      */
     private function calculateLabels($unit)
     {
-        $fac = DateTimeFactory::create();
+        $fac = new DateTime();
 
         $duration = $this->getMax() - $this->getMin();