DateTimeWidgetBase sets Timezone twice to the same value

Problem/Motivation

DateTimeWidgetBase sets timezone twice to the same value/creates unnecessary objects/minor bloat.

...
    if ($items[$delta]->date) {
      $date = $items[$delta]->date;
      // The date was created and verified during field_load(), so it is safe to
      // use without further inspection.
      $date->setTimezone(new \DateTimeZone($element['value']['#date_timezone']));
      $element['value']['#default_value'] = $this->createDefaultValue($date, $element['value']['#date_timezone']);
    }
...
  protected function createDefaultValue($date, $timezone) {
    // The date was created and verified during field_load(), so it is safe to
    // use without further inspection.
    if ($this->getFieldSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
      $date->setDefaultDateTime();
    }
    $date->setTimezone(new \DateTimeZone($timezone));
    return $date;
  }

Proposed resolution

Clean up duplication

Remaining tasks

None

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None