Uninstalling from Joomla
To uninstall Joomdle from Joomla, search for "joomdle" in extension manager, and check all the resutls. Then, unsitall them all at once.
Uninstalling from Moodle
To uninstall Joomdle from Moodle, go to Moodle->Site administration->Plugins->Plugins overview. Look for Joomdle and click Uninstall.
If you don't see Uninstall link, it means that you still have some users with Joomdle as their auth method. You will need to change them to Manual auth method before you can uninstall Joomdle.