Hi is there a smart service that allows to delete a file by FTP? (in the F

There isn't one to my best knowledge but you can create a function plug-in by using the Apache FTP client and the following code.
FTPClient client = new FTPClient();
client.connect(host, port);
client.login(loginname, password);
client.deleteFile(fileNameOnServer);
client.disconnect();

Since there is no direct dependency on any of the Appian services, you would require the URL of the file in the FTP as a parameter and return a Boolean if the service was successful.

You can extend the Remote Transfer plugin code and add another smart service module to the plugin. This should not be a function since it has a "side effect" of deleting a file (unless you make a "writer" function).