How to delete Birthdays from Google Calendar? There is no link in Calendars settings, which allows to unsubscribe from Birthdays calendar in Google Calendar.
Unfortunately, it seems you can no longer unsubscribe from the Birthdays calendar.1
The best you can do is not display it.2 Find it in the list of calendars to the left and click it so the colored square blanks out. That indicates that the events for that calendar will not be displayed in any calendar views. As long as you don't turn it on (and can ignore the unused entry in the list of calendars) it will never bother you again.
Commentary: I don't understand this move by Google. This is something that Facebook would do. Certainly there must be people who don't want to use their calendar to keep track of birthdays and anniversaries.
1 Since the description says "Displays birthdays of people in your Google+ circles and Google Contacts. Also displays anniversary and other event dates from Google Contacts, if applicable." I suppose it might still be possible to unsubscribe if you don't have a Google+ profile, but I doubt it.
2 Your other option would be to go into all of your contacts and remove any dates in their birthday or anniversary fields, and also remove all people from your Google+ Circles. Not very practical.