block_base: config_save needs to marked as deprecated

block_base->config_save does not seem used anywhere any more. It needs to be marked as deprecated in the code then, and wiki changes needs to be made. If contributed plugins rely on it, it makes no difference since it is not being called (so their code will not be executed), if they do something like $this->config_save or parent::config_save somewhere, this will still work as deprecated function will handle it.