Look in Site Admin > Users > Accounts > User Profile fields, you should see it there and be able to delete it. If it is not there, then someone has likely hard coded it into your Moodle. Update your Moodle to a more recent version and that should get rid of it. You may have to access your database via phpMyAdmin or similar and manually delete the field and data via an SQL statement.
General help
Ethnicity fields
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