Fix unused private $tz, add missing $utc private properties

[adrian-martinez-interactiv4]

In code constructor, $this->utc referenced a non-existing property; at the same time, a DateTimeZone private property $tz was declared but not used.

It seems property got renamed somewhere in time in code usage, but not in property declaration.

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• Are you fixing a bug?

  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
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• Are you creating a new feature?

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• Is this related to quality assurance?

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[michalbundyra]

Thanks, @adrian-martinez-interactiv4!