You can uninstall the MailTracker extension from your Chrome's Extension page or by clicking here.
To access the Extensions page in Chrome, click on the three dots on the right-hand side and select Extensions—Manage Extension. Then, scroll through your extensions to find MailTracker: Email tracker for Gmail.
You can disable the extension by toggling it, or if you want to remove it, click the Remove button next to it.
If you need to reinstall it again in the future, head here.