Many iPhone X and other iPhone users are facing issue that after upgrade to iOS 11 they can't remove a calendar. The Edit button that used to be in iOS10 at the left of Calendars seems to missing from subscribed calendar and users are facing issue to remove calendar from iOS11.
Actually many users are facing issue of man holidays missing from their calendar from iOS11 and to get those holiday to their iPhone, alternatively subscribed to calendars.
The answer is the setting to delete a calendar from iOS11 is changed now. In iOS11 to delete a subscribed calendar Go to settings -> accounts & passwords -> subscribed calendars. There, you can find option to delete the calendar.
For older iOS10 Users:
You can directly delete the calendars by go to settings -> calendar -> accounts.