Remove deprecated conditional in Date

Author: josevalim

lib/elixir/lib/calendar/date.ex

@@ -1073,13 +1073,7 @@ defmodule Date do
   def year_of_era(date)
 
   def year_of_era(%{calendar: calendar, year: year, month: month, day: day}) do
-    # TODO: Remove me on 1.17
-    # The behaviour implementation already warns on missing callback.
-    if function_exported?(calendar, :year_of_era, 3) do
-      calendar.year_of_era(year, month, day)
-    else
-      calendar.year_of_era(year)
-    end
+    calendar.year_of_era(year, month, day)
   end
 
   @doc """