Improve preloading request code (#11007)

This removes the conditional branching based on the existence of the `get_compact_response_links()` method on the `$server` object, as that method was introduced with WordPress 4.5, and Gutenberg requires WordPress 4.9.8 or later.

Also, it turns the indirect and slow call through `call_user_func()` into a direct method call.

Note: The call would actually be more correct as a static call (`$server::get_compact_response_links( $response)`), but this notation is PHP 5.3+ only.

lib/client-assets.php

@@ -863,11 +863,7 @@ function gutenberg_preload_api_request( $memo, $path ) {
 	if ( 200 === $response->status ) {
 		$server = rest_get_server();
 		$data   = (array) $response->get_data();
-		if ( method_exists( $server, 'get_compact_response_links' ) ) {
-			$links = call_user_func( array( $server, 'get_compact_response_links' ), $response );
-		} else {
-			$links = call_user_func( array( $server, 'get_response_links' ), $response );
-		}
+		$links  = $server->get_compact_response_links( $response );
 		if ( ! empty( $links ) ) {
 			$data['_links'] = $links;
 		}