[feature/avatars] Move avatar specific settings to drivers

PHPBB3-10018

phpBB/includes/avatar/driver/driver.php

@@ -126,6 +126,14 @@ abstract class phpbb_avatar_driver implements phpbb_avatar_driver_interface
 		return false;
 	}
 
+	/**
+	* @inheritdoc
+	**/
+	public function prepare_form_acp()
+	{
+		return array();
+	}
+
 	/**
 	* @inheritdoc
 	**/