Matomo / Piwik Cookies

Hi,
I have a Matomo installation that I was using on a Wordpress site via your plugin. However, after deciding not to track the site at all, I have deleted the plugin. Trouble is when I look at GDPR compliance, the cookies are still there. In particular there are 2: -
_pk_id_123456
_pk_ses_123456
It looks like I should be able to deregister these in the functions file, but I’d prefer to find out where they are coming from and just delete that code.
Have you any ideas where I should look to find this? I’ve looked in all the usual places, but can’t find it at all. I’ve also cleaned up the database and again it is still there.
Any help would be much appreciated.
Thanks