Remove deprecated Alert#authenticate

It has been deprecated since 3.5.2 and is not supported by any drivers.

Author: p0deje

rb/lib/selenium/webdriver/common/alert.rb

@@ -40,12 +40,6 @@ def send_keys(keys)
       def text
         @bridge.alert_text
       end
-
-      def authenticate(username, password)
-        WebDriver.logger.deprecate 'Alert#authenticate'
-        @bridge.authentication(username: username, password: password)
-        accept
-      end
     end # Alert
   end # WebDriver
 end # Selenium