According to Uninstall directions / possibly critical bug in release 2017021600 · Issue #90 · catalyst/moodle-auth_saml2 · GitHub "Moodle will not let you uninstall it if there is still users mapped to an auth plugin"
On the Manage authentication page /admin/settings.php?section=manageauths it shows one user of SAML2
How do I look up a users authentication method?