Here's how you can delete a calendar inside the Dashboard in AddEvent. Locate the calendar you want to delete in the list of calendars.

Click the "...", a context menu appears, click the "Calendar page" option.

On the calendar page, locate the "Actions" menu on the top right. Click the option "Delete" to delete the calendar.

Please note: If the "Delete" option is grayed out then your calendar is the "Primary calendar". The "Primary calendar" can't be deleted. You can set a different calendar as your "Primary calendar" which will enable the option to delete the calendar.