dateHelperVCalendar.js: revert dayjs.tz back again to dayjs

This was only needed to run the tests in multiple timezones.
Now this is not needed anymore.
(in a later pr we may export dayjs.tz instead of the dayjs object
which does not allow to change the timezone between tests).

common/helpers/dateHelperVCalendar.js

@@ -2,12 +2,12 @@ import dayjs from './dayjs.js'
 
 // converts a timestamp (local timezone) into ISO String format (UTC timezone)
 function timestampToUtcString(timestamp) {
-  return dayjs.utc(dayjs.tz(timestamp).format('YYYY-MM-DD HH:mm')).format()
+  return dayjs.utc(dayjs(timestamp).format('YYYY-MM-DD HH:mm')).format()
 }
 
 // converts ISO String format (UTC timezone) into a timestamp (local timezone)
 function utcStringToTimestamp(string) {
-  return dayjs.tz(dayjs.utc(string).format('YYYY-MM-DD HH:mm')).valueOf()
+  return dayjs(dayjs.utc(string).format('YYYY-MM-DD HH:mm')).valueOf()
 }
 
 export { timestampToUtcString, utcStringToTimestamp }