Here are the steps required to remove the branding.

1)

Open your installation of Kunena, and make sure, you create backup of your site installation first.

2)

Open the following file:
libraries/kunena/src/Controller/Application/Display.php

3)

Find the part of the code containing the following statement:
if ($this->config->get('credits', 1)) {
    $this->output->appendAfter($this->poweredBy());
}